added a comment, improved error message for unresolveable adjacencies,

a sort function is now part of this module, so it can be used standing
alone

dfs.scm

@@ -1,3 +1,28 @@
+;;; 
+;;; merge(?) sort for general purpose:
+;;; ==================================
+;;; 
+;;; (sort predicate list-to-be-sorted to-sort-in-list) ---> sorted-list
+;;; 
+;;; where 
+;;; 
+;;; predicate         : (lambda (a b) ...) with a x b ---> {#t, #f}
+;;;                     e.g. (lambda (a b) (> a b))
+;;; list-to-be-sorted : e.g. '(4 2 5 1 3)
+;;; to-sort-in-list   : e.g. '(6)
+;;; 
+;;; will produce the result '(6 5 4 3 2 1).
+;;; 
+(define (sort pred todo done)
+  (if (null? todo)
+      done
+      (sort pred (cdr todo) (insert pred (car todo) done))))
+
+(define (insert pred item ls)
+  (if (or (null? ls) (pred item (car ls)))
+      (cons item ls)
+      (cons (car ls) (insert pred item (cdr ls)))))
+
 (define time 0)
 
 (define-enumerated-type color :color
@@ -59,7 +84,8 @@
 ;;; 
 ;;; where 
 ;;; 
-;;; dag           : '(#{:dfs} ...)
+;;; dag           : '(#{:dfs} ...) ; representation of a given
+;;;                                  directed acyclic graph) 
 ;;; auto-leafs?   : #t (by default) or #f
 ;;; sorted-dag    : the sorted dag
 ;;; 
@@ -131,7 +157,8 @@
 						   (color white) 0 #f)))
 			(set! current-dag (dfs-visit (cons leaf current-dag) 
 						     leaf auto-leafs?)))
-		      (error "dfs-visit: incomplete dag!"))))
+		      (error "dfs-visit: unresolveable adjacency: "
+			     current-adj))))
 	    (if (not (null? todo-adjs))
 		(for-all-adjs (car todo-adjs) (cdr todo-adjs)))))
       (set! time (+ time 1))