TODO for (R6RS BASE)

* Compiler + libraries:
  - letrec and letrec* restrictions (references and multiple returns)
  - prohibit export of mutable bindings
  - library versioning
  - library phases (simply ignore)
  - internal imports
  - Recognize (define x)
  - Add let-syntax and letrec-syntax
  - Add identifier-syntax
  - Add do, let*-values.

* Numerics:
  - Add rational, complex, and single-precision numbers
  - Make sure the following primitives work:
    
    rationalize

    make-rectangular    make-polar 
    real-part           imag-part
    magnitude           angle

    expt        log        
    sin         cos         tan
    asin        acos        atan

    sqrt    exact-integer-sqrt

    number?   complex?   real?   rational?  integer?  
    real-valued?  rational-valued?   integer-values?
    
    exact?     inexact?    ->exact   ->inexact

    real->flonum    real->single    real->double
    
    =   <   <=   >   >=
    zero?  positive?  negative?
    odd?  even?
    finite?  infinite?  nan?
    min   max   abs

    +       -      *      /
    
    div         mod           div-and-mod
    div0        mod0          div0-and-mod0

    gcd     lcm  (don't deal with inexact ints yet)

    floor       ceiling      truncate    round         

    number->string(radix,precision)    string->number(radix)

* Read:
  - recognize rational, complex and flonum syntax
  - recognize inline-hex sequences (strings, chars, and symbols)
  - #!r6rs
  - #\ sequnces: nul, alarm, backspace, tab, linefeed, vtab, page,
    return, esc, space, delete
  - respect unicode constituents
  - recognize \a, \b, \t, \n, \v, \f, \r, \", \\, \<lf>, \<spc>,
    \xXXX; sequences in strings.
  - Add unsyntax, unsyntax-splicing, and quasisyntax.

* Bytevectors:
  - equal? for bytevectors

* Strings:
  - string-copy    
  - string-fill!

* Vectors:
  - vector-fill!
  - vector-map
  - vector-for-each

* Errors:
  - (error who msg irritants ...)
  - (assertion-violation who message irritants ...)






Completed for (R6RS BASE):
    numerator           denominator