Needed:

    - Warnings on domain errors (see "Domain Errors" in SPSS manual)
      and documentation of such.

    - Finish polishing code.  Many functions need comments.

    - Test the remaining statistical distributions.

    - Implement unimplemented functions.

    - Check treatment of 0 bytes in expressions is correct.

Extension ideas:

    - Short-circuit evaluation of logical ops

    - Conditional operator with ? :

    - User-defined functions.