diff --git a/scheme/more-interfaces.scm b/scheme/more-interfaces.scm
index 14b1dd9..ff2981b 100644
--- a/scheme/more-interfaces.scm
+++ b/scheme/more-interfaces.scm
@@ -648,3 +648,23 @@
    option-optional-arg?
    option-processor
    args-fold))
+
+(define-interface srfi-42-interface
+  (export ((do-ec
+	    list-ec append-ec
+	    string-ec string-append-ec
+	    vector-ec vector-of-length-ec
+	    sum-ec product-ec
+	    min-ec max-ec
+	    any?-ec every?-ec
+	    first-ec last-ec
+	    fold-ec fold3-ec) :syntax)
+	  ((:
+	    :list :string :vector
+	    :integers
+	    :range :real-range :char-range
+	    :port
+	    :dispatched) :syntax)
+	  ((:do :let :parallel :while :until) :syntax)
+	  :-dispatch-ref :-dispatch-set! make-initial-:-dispatch 
+	  (:generator-proc :syntax)))
diff --git a/scheme/more-packages.scm b/scheme/more-packages.scm
index df439f2..cf8022d 100644
--- a/scheme/more-packages.scm
+++ b/scheme/more-packages.scm
@@ -729,8 +729,9 @@
     (define available-srfis
       '(srfi-1 srfi-2 srfi-5 srfi-6 srfi-7 srfi-8 srfi-9
 	srfi-11 srfi-13 srfi-14 srfi-16 srfi-17 srfi-19 srfi-23
-	srfi-25 srfi-26 srfi-27 srfi-28 srfi-30 srfi-31
-	srfi-37))
+	srfi-25 srfi-26 srfi-27 srfi-28 
+        srfi-30 srfi-31	srfi-37
+        srfi-42))
 
     ; Some SRFI's redefine Scheme variables.
     (define shadowed
@@ -873,6 +874,12 @@
 	srfi-11)
   (files (srfi srfi-37)))
 
+; Eager Comprehensions
+
+(define-structure srfi-42 srfi-42-interface
+  (open scheme
+	srfi-23)
+  (files (srfi srfi-42)))
 ; ... end of package definitions.
 
 ; Temporary compatibility stuff
@@ -960,6 +967,7 @@
 	    srfi-11 srfi-13 srfi-14 srfi-16 srfi-17
 	    srfi-23 srfi-25 srfi-26 srfi-27 srfi-28
 	    srfi-31 srfi-37
+            srfi-42
 	    )
 	   :structure)
           make-srfi-19
diff --git a/scheme/srfi/srfi-42.scm b/scheme/srfi/srfi-42.scm
new file mode 100644
index 0000000..f9c678b
--- /dev/null
+++ b/scheme/srfi/srfi-42.scm
@@ -0,0 +1,956 @@
+; <PLAINTEXT>
+; Eager Comprehensions in [outer..inner|expr]-Convention
+; ======================================================
+;
+; sebastian.egner@philips.com, Eindhoven, The Netherlands, Feb-2003.
+; Scheme R5RS (incl. macros), SRFI-23 (error).
+; 
+; Loading the implementation into Scheme48 0.57:
+;   ,open srfi-23
+;   ,load ec.scm
+;
+; Loading the implementation into PLT/DrScheme 202:
+;   ; File > Open ... "ec.scm", click Execute
+;
+; Loading the implementation into SCM 5d7:
+;   (require 'macro) (require 'record) 
+;   (load "ec.scm")
+;
+; Implementation comments:
+;   * All local (not exported) identifiers are named ec-<something>.
+;   * This implementation focuses on portability, performance, 
+;     readability, and simplicity roughly in this order. Design
+;     decisions related to performance are taken for Scheme48.
+;   * Alternative implementations, Comments and Warnings are 
+;     mentioned after the definition with a heading.
+
+
+; ==========================================================================
+; The fundamental comprehension do-ec
+; ==========================================================================
+;
+; All eager comprehensions are reduced into do-ec and
+; all generators are reduced to :do. 
+;
+; We use the following short names for syntactic variables
+;   q    - qualifier
+;   cc   - current continuation, thing to call at the end;
+;          the CPS is (m (cc ...) arg ...) -> (cc ... expr ...)
+;   cmd  - an expression being evaluated for its side-effects
+;   expr - an expression
+;   gen  - a generator of an eager comprehension
+;   ob   - outer binding
+;   oc   - outer command
+;   lb   - loop binding
+;   ne1? - not-end1? (before the payload)
+;   ib   - inner binding
+;   ic   - inner command
+;   ne2? - not-end2? (after the payload)
+;   ls   - loop step
+;   etc  - more arguments of mixed type
+
+
+; (do-ec q ... cmd)
+;   handles nested, if/not/and/or, begin, :let, and calls generator 
+;   macros in CPS to transform them into fully decorated :do.
+;   The code generation for a :do is delegated to do-ec:do.
+
+(define-syntax do-ec
+  (syntax-rules (nested if not and or begin :do let)
+
+    ; explicit nesting -> implicit nesting
+    ((do-ec (nested q ...) etc ...)
+     (do-ec q ... etc ...) )
+
+    ; implicit nesting -> fold do-ec
+    ((do-ec q1 q2 etc1 etc ...)
+     (do-ec q1 (do-ec q2 etc1 etc ...)) )
+
+    ; no qualifiers at all -> evaluate cmd once
+    ((do-ec cmd)
+     (begin cmd (if #f #f)) )
+
+; now (do-ec q cmd) remains
+
+    ; filter -> make conditional
+    ((do-ec (if test) cmd)
+     (if test (do-ec cmd)) )
+    ((do-ec (not test) cmd)
+     (if (not test) (do-ec cmd)) )
+    ((do-ec (and test ...) cmd)
+     (if (and test ...) (do-ec cmd)) )
+    ((do-ec (or test ...) cmd)
+     (if (or test ...) (do-ec cmd)) )
+
+    ; begin -> make a sequence
+    ((do-ec (begin etc ...) cmd)
+     (begin etc ... (do-ec cmd)) )
+
+    ; fully decorated :do-generator -> delegate to do-ec:do
+    ((do-ec (:do olet lbs ne1? ilet ne2? lss) cmd)
+     (do-ec:do cmd (:do olet lbs ne1? ilet ne2? lss)) )
+
+; anything else -> call generator-macro in CPS; reentry at (*)
+
+    ((do-ec (g arg1 arg ...) cmd)
+     (g (do-ec:do cmd) arg1 arg ...) )))
+
+
+; (do-ec:do cmd (:do olet lbs ne1? ilet ne2? lss)
+;   generates code for a single fully decorated :do-generator
+;   with cmd as payload, taking care of special cases.
+
+(define-syntax do-ec:do
+  (syntax-rules (:do let)
+
+    ; reentry point (*) -> generate code
+    ((do-ec:do cmd 
+               (:do (let obs oc ...) 
+                    lbs 
+                    ne1? 
+                    (let ibs ic ...) 
+                    ne2? 
+                    (ls ...) ))
+     (ec-simplify
+       (let obs
+         oc ...
+         (let loop lbs
+           (ec-simplify
+             (if ne1?
+                 (ec-simplify
+                   (let ibs
+                      ic ...
+                      cmd
+                      (ec-simplify
+                        (if ne2?
+                            (loop ls ...) )))))))))) ))
+
+    
+; (ec-simplify <expression>)
+;   generates potentially more efficient code for <expression>.
+;   The macro handles if, (begin <command>*), and (let () <command>*)
+;   and takes care of special cases.
+
+(define-syntax ec-simplify
+  (syntax-rules (if not let begin)
+
+; one- and two-sided if
+
+    ; literal <test>
+    ((ec-simplify (if #t consequent))
+     consequent )
+    ((ec-simplify (if #f consequent))
+     (if #f #f) )
+    ((ec-simplify (if #t consequent alternate))
+     consequent )
+    ((ec-simplify (if #f consequent alternate))
+     alternate )
+
+    ; (not (not <test>))
+    ((ec-simplify (if (not (not test)) consequent))
+     (ec-simplify (if test consequent)) )
+    ((ec-simplify (if (not (not test)) consequent alternate))
+     (ec-simplify (if test consequent alternate)) )
+
+; (let () <command>*) 
+
+    ; empty <binding spec>*
+    ((ec-simplify (let () command ...))
+     (ec-simplify (begin command ...)) )
+
+; begin 
+
+    ; flatten use helper (ec-simplify 1 done to-do)
+    ((ec-simplify (begin command ...))
+     (ec-simplify 1 () (command ...)) )
+    ((ec-simplify 1 done ((begin to-do1 ...) to-do2 ...))
+     (ec-simplify 1 done (to-do1 ... to-do2 ...)) )
+    ((ec-simplify 1 (done ...) (to-do1 to-do ...))
+     (ec-simplify 1 (done ... to-do1) (to-do ...)) )
+
+    ; exit helper
+    ((ec-simplify 1 () ())
+     (if #f #f) )
+    ((ec-simplify 1 (command) ())
+     command )
+    ((ec-simplify 1 (command1 command ...) ())
+     (begin command1 command ...) )
+
+; anything else
+
+    ((ec-simplify expression)
+     expression )))
+
+
+; ==========================================================================
+; The special generators :do, :let, :parallel, :while, and :until
+; ==========================================================================
+
+(define-syntax :do
+  (syntax-rules ()
+
+    ; full decorated -> continue with cc, reentry at (*)
+    ((:do (cc ...) olet lbs ne1? ilet ne2? lss)
+     (cc ... (:do olet lbs ne1? ilet ne2? lss)) )
+
+    ; short form -> fill in default values
+    ((:do cc lbs ne1? lss)
+     (:do cc (let ()) lbs ne1? (let ()) #t lss) )))
+    
+
+(define-syntax :let
+  (syntax-rules (index)
+    ((:let cc var (index i) expression)
+     (:do cc (let ((var expression) (i 0))) () #t (let ()) #f ()) )
+    ((:let cc var expression)
+     (:do cc (let ((var expression))) () #t (let ()) #f ()) )))
+
+
+(define-syntax :parallel
+  (syntax-rules (:do)
+    ((:parallel cc)
+     cc )
+    ((:parallel cc (g arg1 arg ...) gen ...)
+     (g (:parallel-1 cc (gen ...)) arg1 arg ...) )))
+
+; (:parallel-1 cc (to-do ...) result [ next ] )
+;    iterates over to-do by converting the first generator into 
+;    the :do-generator next and merging next into result.
+
+(define-syntax :parallel-1  ; used as 
+  (syntax-rules (:do let)
+
+    ; process next element of to-do, reentry at (**)
+    ((:parallel-1 cc ((g arg1 arg ...) gen ...) result)
+     (g (:parallel-1 cc (gen ...) result) arg1 arg ...) )
+
+    ; reentry point (**) -> merge next into result
+    ((:parallel-1 
+       cc 
+       gens 
+       (:do (let (ob1 ...) oc1 ...) 
+            (lb1 ...) 
+            ne1?1 
+            (let (ib1 ...) ic1 ...) 
+            ne2?1 
+            (ls1 ...) )
+       (:do (let (ob2 ...) oc2 ...) 
+            (lb2 ...) 
+            ne1?2 
+            (let (ib2 ...) ic2 ...) 
+            ne2?2 
+            (ls2 ...) ))
+     (:parallel-1 
+       cc 
+       gens 
+       (:do (let (ob1 ... ob2 ...) oc1 ... oc2 ...) 
+            (lb1 ... lb2 ...) 
+            (and ne1?1 ne1?2) 
+            (let (ib1 ... ib2 ...) ic1 ... ic2 ...) 
+            (and ne2?1 ne2?2) 
+            (ls1 ... ls2 ...) )))
+
+    ; no more gens -> continue with cc, reentry at (*)
+    ((:parallel-1 (cc ...) () result)
+     (cc ... result) )))
+
+
+(define-syntax :while
+  (syntax-rules ()
+    ((:while cc (g arg1 arg ...) test)
+     (g (:while-1 cc test) arg1 arg ...) )))
+
+(define-syntax :while-1
+  (syntax-rules (:do)
+    ((:while-1 cc test (:do olet lbs ne1? ilet ne2? lss))
+     (:do cc olet lbs (and ne1? test) ilet ne2? lss) )))
+
+
+(define-syntax :until
+  (syntax-rules ()
+    ((:until cc (g arg1 arg ...) test)
+     (g (:until-1 cc test) arg1 arg ...) )))
+
+(define-syntax :until-1
+  (syntax-rules (:do)
+    ((:until-1 cc test (:do olet lbs ne1? ilet ne2? lss))
+     (:do cc olet lbs ne1? ilet (and ne2? (not test)) lss) )))
+
+
+; ==========================================================================
+; The typed generators :list :string :vector etc.
+; ==========================================================================
+
+(define-syntax :list
+  (syntax-rules (index)
+    ((:list cc var (index i) arg ...)
+     (:parallel cc (:list var arg ...) (:integers i)) )
+    ((:list cc var arg1 arg2 arg ...)
+     (:list cc var (append arg1 arg2 arg ...)) )
+    ((:list cc var arg)
+     (:do cc
+          (let ())
+          ((t arg))
+          (not (null? t))
+          (let ((var (car t))))
+          #t
+          ((cdr t)) ))))
+
+
+(define-syntax :string
+  (syntax-rules (index)
+    ((:string cc var (index i) arg)
+     (:do cc
+          (let ((str arg) (len 0)) 
+            (set! len (string-length str)))
+          ((i 0))
+          (< i len)
+          (let ((var (string-ref str i))))
+          #t
+          ((+ i 1)) ))
+    ((:string cc var (index i) arg1 arg2 arg ...)
+     (:string cc var (index i) (string-append arg1 arg2 arg ...)) )
+    ((:string cc var arg1 arg ...)
+     (:string cc var (index i) arg1 arg ...) )))
+
+; Alternative: An implementation in the style of :vector can also
+;   be used for :string. However, it is less interesting as the
+;   overhead of string-append is much less than for 'vector-append'.
+
+
+(define-syntax :vector
+  (syntax-rules (index)
+    ((:vector cc var arg)
+     (:vector cc var (index i) arg) )
+    ((:vector cc var (index i) arg)
+     (:do cc
+          (let ((vec arg) (len 0)) 
+            (set! len (vector-length vec)))
+          ((i 0))
+          (< i len)
+          (let ((var (vector-ref vec i))))
+          #t
+          ((+ i 1)) ))
+
+    ((:vector cc var (index i) arg1 arg2 arg ...)
+     (:parallel cc (:vector cc var arg1 arg2 arg ...) (:integers i)) )
+    ((:vector cc var arg1 arg2 arg ...)
+     (:do cc
+          (let ((vec #f)
+                (len 0)
+                (vecs (ec-:vector-filter (list arg1 arg2 arg ...))) ))
+          ((k 0))
+          (if (< k len)
+              #t
+              (if (null? vecs)
+                  #f
+                  (begin (set! vec (car vecs))
+                         (set! vecs (cdr vecs))
+                         (set! len (vector-length vec))
+                         (set! k 0)
+                         #t )))
+          (let ((var (vector-ref vec k))))
+          #t
+          ((+ k 1)) ))))
+
+(define (ec-:vector-filter vecs)
+  (if (null? vecs)
+      '()
+      (if (zero? (vector-length (car vecs)))
+          (ec-:vector-filter (cdr vecs))
+          (cons (car vecs) (ec-:vector-filter (cdr vecs))) )))
+
+; Alternative: A simpler implementation for :vector uses vector->list
+;   append and :list in the multi-argument case. Please refer to the
+;   'design.scm' for more details.
+
+
+(define-syntax :integers
+  (syntax-rules (index)
+    ((:integers cc var (index i))
+     (:do cc ((var 0) (i 0)) #t ((+ var 1) (+ i 1))) )
+    ((:integers cc var)
+     (:do cc ((var 0)) #t ((+ var 1))) )))
+
+
+(define-syntax :range
+  (syntax-rules (index)
+
+    ; handle index variable and add optional args
+    ((:range cc var (index i) arg1 arg ...)
+     (:parallel cc (:range var arg1 arg ...) (:integers i)) )
+    ((:range cc var arg1)
+     (:range cc var 0 arg1 1) )
+    ((:range cc var arg1 arg2)
+     (:range cc var arg1 arg2 1) )
+
+; special cases (partially evaluated by hand from general case)
+
+    ((:range cc var 0 arg2 1)
+     (:do cc
+          (let ((b arg2))
+            (if (not (and (integer? b) (exact? b)))
+                (error 
+                   "arguments of :range are not exact integer "
+                   "(use :real-range?)" 0 b 1 )))
+          ((var 0))
+          (< var b)
+          (let ())
+          #t
+          ((+ var 1)) ))
+
+    ((:range cc var 0 arg2 -1)
+     (:do cc
+          (let ((b arg2))
+            (if (not (and (integer? b) (exact? b)))
+                (error 
+                   "arguments of :range are not exact integer "
+                   "(use :real-range?)" 0 b 1 )))
+          ((var 0))
+          (> var b)
+          (let ())
+          #t
+          ((- var 1)) ))
+
+    ((:range cc var arg1 arg2 1)
+     (:do cc
+          (let ((a arg1) (b arg2))
+            (if (not (and (integer? a) (exact? a)
+                          (integer? b) (exact? b) ))
+                (error 
+                   "arguments of :range are not exact integer "
+                   "(use :real-range?)" a b 1 )) )
+          ((var a))
+          (< var b)
+          (let ())
+          #t
+          ((+ var 1)) ))
+
+    ((:range cc var arg1 arg2 -1)
+     (:do cc
+          (let ((a arg1) (b arg2) (s -1) (stop 0))
+            (if (not (and (integer? a) (exact? a)
+                          (integer? b) (exact? b) ))
+                (error 
+                   "arguments of :range are not exact integer "
+                   "(use :real-range?)" a b -1 )) )
+          ((var a))
+          (> var b)
+          (let ())
+          #t
+          ((- var 1)) ))
+
+; the general case
+
+    ((:range cc var arg1 arg2 arg3)
+     (:do cc
+          (let ((a arg1) (b arg2) (s arg3) (stop 0))
+            (if (not (and (integer? a) (exact? a)
+                          (integer? b) (exact? b)
+                          (integer? s) (exact? s) ))
+                (error 
+                   "arguments of :range are not exact integer "
+                   "(use :real-range?)" a b s ))
+            (if (zero? s)
+                (error "step size must not be zero in :range") )
+            (set! stop (+ a (* (max 0 (ceiling (/ (- b a) s))) s))) )
+          ((var a))
+          (not (= var stop))
+          (let ())
+          #t
+          ((+ var s)) ))))
+
+; Comment: The macro :range inserts some code to make sure the values
+;   are exact integers. This overhead has proven very helpful for 
+;   saving users from themselves.
+
+
+(define-syntax :real-range
+  (syntax-rules (index)
+
+    ; add optional args and index variable
+    ((:real-range cc var arg1)
+     (:real-range cc var (index i) 0 arg1 1) )
+    ((:real-range cc var (index i) arg1)
+     (:real-range cc var (index i) 0 arg1 1) )
+    ((:real-range cc var arg1 arg2)
+     (:real-range cc var (index i) arg1 arg2 1) )
+    ((:real-range cc var (index i) arg1 arg2)
+     (:real-range cc var (index i) arg1 arg2 1) )
+    ((:real-range cc var arg1 arg2 arg3)
+     (:real-range cc var (index i) arg1 arg2 arg3) )
+
+    ; the fully qualified case
+    ((:real-range cc var (index i) arg1 arg2 arg3)
+     (:do cc
+          (let ((a arg1) (b arg2) (s arg3) (istop 0))
+            (if (not (and (real? a) (real? b) (real? s)))
+                (error "arguments of :real-range are not real" a b s) )
+            (if (and (exact? a) (or (not (exact? b)) (not (exact? s))))
+                (set! a (exact->inexact a)) )
+            (set! istop (/ (- b a) s)) )
+          ((i 0))
+          (< i istop)
+          (let ((var (+ a (* s i)))))
+          #t
+          ((+ i 1)) ))))
+
+; Comment: The macro :real-range adapts the exactness of the start
+;   value in case any of the other values is inexact. This is a
+;   precaution to avoid (list-ec (: x 0 3.0) x) => '(0 1.0 2.0).
+
+    
+(define-syntax :char-range
+  (syntax-rules (index)
+    ((:char-range cc var (index i) arg1 arg2)
+     (:parallel cc (:char-range var arg1 arg2) (:integers i)) )
+    ((:char-range cc var arg1 arg2)
+     (:do cc
+          (let ((imax (char->integer arg2))))
+          ((i (char->integer arg1)))
+          (<= i imax)
+          (let ((var (integer->char i))))
+          #t
+          ((+ i 1)) ))))
+
+; Warning: There is no R5RS-way to implement the :char-range generator 
+;   because the integers obtained by char->integer are not necessarily 
+;   consecutive. We simply assume this anyhow for illustration.
+
+
+(define-syntax :port
+  (syntax-rules (index)
+    ((:port cc var (index i) arg1 arg ...)
+     (:parallel cc (:port var arg1 arg ...) (:integers i)) )
+    ((:port cc var arg)
+     (:port cc var arg read) )
+    ((:port cc var arg1 arg2)
+     (:do cc
+          (let ((port arg1) (read-proc arg2)))
+          ((var (read-proc port)))
+          (not (eof-object? var))
+          (let ())
+          #t
+          ((read-proc port)) ))))
+
+
+; ==========================================================================
+; The typed generator :dispatched and utilities for constructing dispatchers
+; ==========================================================================
+
+(define-syntax :dispatched
+  (syntax-rules (index)
+    ((:dispatched cc var (index i) dispatch arg1 arg ...)
+     (:parallel cc 
+                (:integers i)
+                (:dispatched var dispatch arg1 arg ...) ))
+    ((:dispatched cc var dispatch arg1 arg ...)
+     (:do cc
+          (let ((d dispatch) 
+                (args (list arg1 arg ...)) 
+                (g #f) 
+                (empty (list #f)) )
+            (set! g (d args))
+            (if (not (procedure? g))
+                (error "unrecognized arguments in dispatching" 
+                       args 
+                       (d '()) )))
+          ((var (g empty)))
+          (not (eq? var empty))
+          (let ())
+          #t
+          ((g empty)) ))))
+
+; Comment: The unique object empty is created as a newly allocated
+;   non-empty list. It is compared using eq? which distinguishes
+;   the object from any other object, according to R5RS 6.1.
+
+
+(define-syntax :generator-proc
+  (syntax-rules (:do let)
+
+    ; call g with a variable, reentry at (**)
+    ((:generator-proc (g arg ...))
+     (g (:generator-proc var) var arg ...) )
+
+    ; reentry point (**) -> make the code from a single :do
+    ((:generator-proc
+       var 
+       (:do (let obs oc ...) 
+            ((lv li) ...) 
+            ne1? 
+            (let ((i v) ...) ic ...) 
+            ne2? 
+            (ls ...)) )
+     (ec-simplify 
+      (let obs
+          oc ...
+          (let ((lv li) ... (ne2 #t))
+            (ec-simplify
+             (let ((i #f) ...) ; v not yet valid
+               (lambda (empty)
+                 (if (and ne1? ne2)
+                     (ec-simplify
+                      (begin 
+                        (set! i v) ...
+                        ic ...
+                        (let ((value var))
+                          (ec-simplify
+                           (if ne2?
+                               (ec-simplify 
+                                (begin (set! lv ls) ...) )
+                               (set! ne2 #f) ))
+                          value )))
+                     empty ))))))))
+
+    ; silence warnings of some macro expanders
+    ((:generator-proc var)
+     (error "illegal macro call") )))
+
+
+(define (dispatch-union d1 d2)
+  (lambda (args)
+    (let ((g1 (d1 args)) (g2 (d2 args)))
+      (if g1
+          (if g2 
+              (if (null? args)
+                  (append (if (list? g1) g1 (list g1)) 
+                          (if (list? g2) g2 (list g2)) )
+                  (error "dispatching conflict" args (d1 '()) (d2 '())) )
+              g1 )
+          (if g2 g2 #f) ))))
+
+
+; ==========================================================================
+; The dispatching generator :
+; ==========================================================================
+
+(define (make-initial-:-dispatch)
+  (lambda (args)
+    (case (length args)
+      ((0) 'SRFI42)
+      ((1) (let ((a1 (car args)))
+             (cond
+              ((list? a1)
+               (:generator-proc (:list a1)) )
+              ((string? a1)
+               (:generator-proc (:string a1)) )
+              ((vector? a1)
+               (:generator-proc (:vector a1)) )
+              ((and (integer? a1) (exact? a1))
+               (:generator-proc (:range a1)) )
+              ((real? a1)
+               (:generator-proc (:real-range a1)) )
+              ((input-port? a1)
+               (:generator-proc (:port a1)) )
+              (else
+               #f ))))
+      ((2) (let ((a1 (car args)) (a2 (cadr args)))
+             (cond
+              ((and (list? a1) (list? a2))
+               (:generator-proc (:list a1 a2)) )
+              ((and (string? a1) (string? a1))
+               (:generator-proc (:string a1 a2)) )
+              ((and (vector? a1) (vector? a2))
+               (:generator-proc (:vector a1 a2)) )
+              ((and (integer? a1) (exact? a1) (integer? a2) (exact? a2))
+               (:generator-proc (:range a1 a2)) )
+              ((and (real? a1) (real? a2))
+               (:generator-proc (:real-range a1 a2)) )
+              ((and (char? a1) (char? a2))
+               (:generator-proc (:char-range a1 a2)) )
+              ((and (input-port? a1) (procedure? a2))
+               (:generator-proc (:port a1 a2)) )
+              (else
+               #f ))))
+      ((3) (let ((a1 (car args)) (a2 (cadr args)) (a3 (caddr args)))
+             (cond
+              ((and (list? a1) (list? a2) (list? a3))
+               (:generator-proc (:list a1 a2 a3)) )
+              ((and (string? a1) (string? a1) (string? a3))
+               (:generator-proc (:string a1 a2 a3)) )
+              ((and (vector? a1) (vector? a2) (vector? a3))
+               (:generator-proc (:vector a1 a2 a3)) )
+              ((and (integer? a1) (exact? a1) 
+                    (integer? a2) (exact? a2)
+                    (integer? a3) (exact? a3))
+               (:generator-proc (:range a1 a2 a3)) )
+              ((and (real? a1) (real? a2) (real? a3))
+               (:generator-proc (:real-range a1 a2 a3)) )
+              (else
+               #f ))))
+      (else
+       (letrec ((every? 
+                 (lambda (pred args)
+                   (if (null? args)
+                       #t
+                       (and (pred (car args))
+                            (every? pred (cdr args)) )))))
+         (cond
+          ((every? list? args)
+           (:generator-proc (:list (apply append args))) )
+          ((every? string? args)
+           (:generator-proc (:string (apply string-append args))) )
+          ((every? vector? args)
+           (:generator-proc (:list (apply append (map vector->list args)))) )
+          (else
+           #f )))))))
+
+(define :-dispatch
+  (make-initial-:-dispatch) )
+
+(define (:-dispatch-ref)
+  :-dispatch )
+
+(define (:-dispatch-set! dispatch)
+  (if (not (procedure? dispatch))
+      (error "not a procedure" dispatch) )
+  (set! :-dispatch dispatch) )
+
+(define-syntax :
+  (syntax-rules (index)
+    ((: cc var (index i) arg1 arg ...)
+     (:dispatched cc var (index i) :-dispatch arg1 arg ...) )
+    ((: cc var arg1 arg ...)
+     (:dispatched cc var :-dispatch arg1 arg ...) )))
+
+
+; ==========================================================================
+; The utility comprehensions fold-ec, fold3-ec
+; ==========================================================================
+
+(define-syntax fold3-ec
+  (syntax-rules (nested)
+    ((fold3-ec x0 (nested q1 ...) q etc1 etc2 etc3 etc ...)
+     (fold3-ec x0 (nested q1 ... q) etc1 etc2 etc3 etc ...) )
+    ((fold3-ec x0 q1 q2 etc1 etc2 etc3 etc ...)
+     (fold3-ec x0 (nested q1 q2) etc1 etc2 etc3 etc ...) )
+    ((fold3-ec x0 expression f1 f2)
+     (fold3-ec x0 (nested) expression f1 f2) )
+
+    ((fold3-ec x0 qualifier expression f1 f2)
+     (let ((result #f) (empty #t))
+       (do-ec qualifier
+              (let ((value expression)) ; don't duplicate
+                (if empty
+                    (begin (set! result (f1 value))
+                           (set! empty #f) )
+                    (set! result (f2 value result)) )))
+       (if empty x0 result) ))))
+
+
+(define-syntax fold-ec
+  (syntax-rules (nested)
+    ((fold-ec x0 (nested q1 ...) q etc1 etc2 etc ...)
+     (fold-ec x0 (nested q1 ... q) etc1 etc2 etc ...) )
+    ((fold-ec x0 q1 q2 etc1 etc2 etc ...)
+     (fold-ec x0 (nested q1 q2) etc1 etc2 etc ...) )
+    ((fold-ec x0 expression f2)
+     (fold-ec x0 (nested) expression f2) )
+
+    ((fold-ec x0 qualifier expression f2)
+     (let ((result x0))
+       (do-ec qualifier (set! result (f2 expression result)))
+       result ))))
+
+
+; ==========================================================================
+; The comprehensions list-ec string-ec vector-ec etc.
+; ==========================================================================
+
+(define-syntax list-ec
+  (syntax-rules ()
+    ((list-ec etc1 etc ...)
+     (reverse (fold-ec '() etc1 etc ... cons)) )))
+
+; Alternative: Reverse can safely be replaced by reverse! if you have it.
+;
+; Alternative: It is possible to construct the result in the correct order
+;   using set-cdr! to add at the tail. This removes the overhead of copying
+;   at the end, at the cost of more book-keeping.
+
+
+(define-syntax append-ec
+  (syntax-rules ()
+    ((append-ec etc1 etc ...)
+     (apply append (list-ec etc1 etc ...)) )))
+
+(define-syntax string-ec
+  (syntax-rules ()
+    ((string-ec etc1 etc ...)
+     (list->string (list-ec etc1 etc ...)) )))
+
+; Alternative: For very long strings, the intermediate list may be a
+;   problem. A more space-aware implementation collect the characters 
+;   in an intermediate list and when this list becomes too large it is
+;   converted into an intermediate string. At the end, the intermediate
+;   strings are concatenated with string-append.
+
+
+(define-syntax string-append-ec
+  (syntax-rules ()
+    ((string-append-ec etc1 etc ...)
+     (apply string-append (list-ec etc1 etc ...)) )))
+
+(define-syntax vector-ec
+  (syntax-rules ()
+    ((vector-ec etc1 etc ...)
+     (list->vector (list-ec etc1 etc ...)) )))
+
+; Comment: A similar approach as for string-ec can be used for vector-ec.
+;   However, the space overhead for the intermediate list is much lower
+;   than for string-ec and as there is no vector-append, the intermediate
+;   vectors must be copied explicitly.
+
+(define-syntax vector-of-length-ec
+  (syntax-rules (nested)
+    ((vector-of-length-ec k (nested q1 ...) q etc1 etc ...)
+     (vector-of-length-ec k (nested q1 ... q) etc1 etc ...) )
+    ((vector-of-length-ec k q1 q2             etc1 etc ...)
+     (vector-of-length-ec k (nested q1 q2)    etc1 etc ...) )
+    ((vector-of-length-ec k expression)
+     (vector-of-length-ec k (nested) expression) )
+
+    ((vector-of-length-ec k qualifier expression)
+     (let ((len k))
+       (let ((vec (make-vector len))
+             (i 0) )
+         (do-ec qualifier
+                (if (< i len)
+                    (begin (vector-set! vec i expression)
+                           (set! i (+ i 1)) )
+                    (error "vector is too short for the comprehension") ))
+         (if (= i len)
+             vec
+             (error "vector is too long for the comprehension") ))))))
+
+
+(define-syntax sum-ec
+  (syntax-rules ()
+    ((sum-ec etc1 etc ...)
+     (fold-ec (+) etc1 etc ... +) )))
+
+(define-syntax product-ec
+  (syntax-rules ()
+    ((product-ec etc1 etc ...)
+     (fold-ec (*) etc1 etc ... *) )))
+
+(define-syntax min-ec
+  (syntax-rules ()
+    ((min-ec etc1 etc ...)
+     (fold3-ec (min) etc1 etc ... min min) )))
+
+(define-syntax max-ec
+  (syntax-rules ()
+    ((max-ec etc1 etc ...)
+     (fold3-ec (max) etc1 etc ... max max) )))
+
+(define-syntax last-ec
+  (syntax-rules (nested)
+    ((last-ec default (nested q1 ...) q etc1 etc ...)
+     (last-ec default (nested q1 ... q) etc1 etc ...) )
+    ((last-ec default q1 q2             etc1 etc ...)
+     (last-ec default (nested q1 q2)    etc1 etc ...) )
+    ((last-ec default expression)
+     (last-ec default (nested) expression) )
+
+    ((last-ec default qualifier expression)
+     (let ((result default))
+       (do-ec qualifier (set! result expression))
+       result ))))
+
+
+; ==========================================================================
+; The fundamental early-stopping comprehension first-ec
+; ==========================================================================
+
+(define-syntax first-ec
+  (syntax-rules (nested)
+    ((first-ec default (nested q1 ...) q etc1 etc ...)
+     (first-ec default (nested q1 ... q) etc1 etc ...) )
+    ((first-ec default q1 q2             etc1 etc ...)
+     (first-ec default (nested q1 q2)    etc1 etc ...) )
+    ((first-ec default expression)
+     (first-ec default (nested) expression) )
+
+    ((first-ec default qualifier expression)
+     (let ((result default) (stop #f))
+       (ec-guarded-do-ec 
+         stop 
+         (nested qualifier)
+         (begin (set! result expression)
+                (set! stop #t) ))
+       result ))))
+
+; (ec-guarded-do-ec stop (nested q ...) cmd)
+;   constructs (do-ec q ... cmd) where the generators gen in q ... are
+;   replaced by (:until gen stop).
+
+(define-syntax ec-guarded-do-ec
+  (syntax-rules (nested if not and or begin)
+
+    ((ec-guarded-do-ec stop (nested (nested q1 ...) q2 ...) cmd)
+     (ec-guarded-do-ec stop (nested q1 ... q2 ...) cmd) )
+
+    ((ec-guarded-do-ec stop (nested (if test) q ...) cmd)
+     (if test (ec-guarded-do-ec stop (nested q ...) cmd)) )
+    ((ec-guarded-do-ec stop (nested (not test) q ...) cmd)
+     (if (not test) (ec-guarded-do-ec stop (nested q ...) cmd)) )
+    ((ec-guarded-do-ec stop (nested (and test ...) q ...) cmd)
+     (if (and test ...) (ec-guarded-do-ec stop (nested q ...) cmd)) )
+    ((ec-guarded-do-ec stop (nested (or test ...) q ...) cmd)
+     (if (or test ...) (ec-guarded-do-ec stop (nested q ...) cmd)) )
+
+    ((ec-guarded-do-ec stop (nested (begin etc ...) q ...) cmd)
+     (begin etc ... (ec-guarded-do-ec stop (nested q ...) cmd)) )
+
+    ((ec-guarded-do-ec stop (nested gen q ...) cmd)
+     (do-ec 
+       (:until gen stop) 
+       (ec-guarded-do-ec stop (nested q ...) cmd) ))
+
+    ((ec-guarded-do-ec stop (nested) cmd)
+     (do-ec cmd) )))
+
+; Alternative: Instead of modifying the generator with :until, it is
+;   possible to use call-with-current-continuation:
+;
+;   (define-synatx first-ec 
+;     ...same as above...
+;     ((first-ec default qualifier expression)
+;      (call-with-current-continuation 
+;       (lambda (cc)
+;        (do-ec qualifier (cc expression))
+;        default ))) ))
+;
+;   This is much simpler but not necessarily as efficient.
+
+
+; ==========================================================================
+; The early-stopping comprehensions any?-ec every?-ec
+; ==========================================================================
+
+(define-syntax any?-ec
+  (syntax-rules (nested)
+    ((any?-ec (nested q1 ...) q etc1 etc ...)
+     (any?-ec (nested q1 ... q) etc1 etc ...) )
+    ((any?-ec q1 q2             etc1 etc ...)
+     (any?-ec (nested q1 q2)    etc1 etc ...) )
+    ((any?-ec expression)
+     (any?-ec (nested) expression) )
+
+    ((any?-ec qualifier expression)
+     (first-ec #f qualifier (if expression) #t) )))
+
+(define-syntax every?-ec
+  (syntax-rules (nested)
+    ((every?-ec (nested q1 ...) q etc1 etc ...)
+     (every?-ec (nested q1 ... q) etc1 etc ...) )
+    ((every?-ec q1 q2             etc1 etc ...)
+     (every?-ec (nested q1 q2)    etc1 etc ...) )
+    ((every?-ec expression)
+     (every?-ec (nested) expression) )
+
+    ((every?-ec qualifier expression)
+     (first-ec #t qualifier (if (not expression)) #f) )))
+