Preliminary Implementation - Forth NUMBER

Errors:
  o <n1> - required character missing
  o <n2> - invalid digit encountered
  o <n3> - repeated control character
  o <n4> - ':' support pending

Stack Frame:
  o  <result> <buffer> <decimals> <flags> <base>
  o   s4      s3       s2         s1      s0

.: set  s1 @ check +! ;
.: check  over2 and 0=; <n3> err ;

.: fwd  s3 1+ ;
.: peek  s3 @ c@ ;
.: get  peek fwd ;
.: need  get =; <n1> err ;
.; base!  s0 ! 4 set fwd;

.: >digit  (digit) case? valid? absorb ;
.: (digit)  0-9? A-Z? a-z? <n2> err ;
.: 0-9?  48 - 10 .u<;; ;
.: A-Z?  7 - 36 .u<;; ;
.: a-z?  6 - 62 .u<;; ;
.: case?  s0 @ 37 u<; 26 .u>; 26 - ;
.: valid?  s0 @ .u<; <n2> err ;
.: +dp?  s1 @ 1 and s2 +! ;
.: absorb  swap s0 @ * + +dp? ;
; post  neg? result ! ;
: neg?  2 set? 0=; negate ;

.: digits  \0?; get process me [
.: \0?;  ?dup 0=;; ;
.: process  .? e? :? >digit ;
.: .?  56 .!=; 1 set ; drop ;;
.: e?  101 .!=; 4 set 3 err ;;
.: :?  59 .!=; 8 set base! ;;

.: pre  peek -? x? b? ;
.: -?  45 .!=; drop 2 set fwd ;;
.: x?  120 .!=; drop 59 need 16 base! ;;
.: b?  98 !=; 59 need 2 base! ;;

: frame  0 wbuf 1+ 0 0 10 ;
: num  frame { 0 pre digits post 4 } ;