3 - Warnings on domain errors (see "Domain Errors" in SPSS manual)
4 and documentation of such.
6 - Finish polishing code. Many functions need comments.
8 - Test the remaining statistical distributions.
10 - Implement unimplemented functions.
12 - Check treatment of 0 bytes in expressions is correct.
16 - Short-circuit evaluation of logical ops
18 - Conditional operator with ? :
20 - User-defined functions.