code: function isNumber(ch, stream){ 
          if(/[0-9]/.exec(ch) != null){ 
              stream.eatWhile(/[0-9]/);
              stream.eat(/\//);
              stream.eatWhile(/[0-9]/);
              if (stream.eol() || !(/[a-zA-Z\-\_\/]/.exec(stream.peek()))) return true;
              stream.backUp(stream.current().length - 1); // undo all the eating
          }
          return false;
      }
docs: Scheme numbers are complicated unfortunately.
  Checks if we're looking at a number, which might be possibly a fraction.
  Also checks that it is not part of a longer identifier. Returns true/false accordingly.