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