3 - Must test vectors on expressions.
5 - Test range syntax (VAR00 to VAR99).
7 - Warnings on domain errors (see "Domain Errors" in SPSS manual)
8 and documentation of such.
10 - Test generic optimizations for correctness.
12 - Update top-level TODO.
14 - Finish polishing code. Many functions need comments.
18 - Short-circuit evaluation of logical ops
20 - Conditional operator with ? :
22 - User-defined functions.