- adjust commented-out evaluation to avoid warnings.

  With code like this
  ```
  #if 0
  nxin    #eval{ round($nxin / 5) };
  #endif
  ```

  The handling of the "#if 0 / #endif" clause uses the plain ISstream
  parser to tokenize. This means that the "round(" is parsed as a word
  with a mismatched closing ')', whereas the "#eval" parser will slurp
  everything in until the closing brace and send it off as a string
  to the expression parser.