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