Scheme program errors without making Rua abort now, although there are a
few things that still need to actually report errors instead of failing
in weird ways.
- Boolean type (no support in lexer yet)
- Conditionals
- Defines (only work correctly at top level)
- More core builtins (apply, cons, car, cdr)
- Variable-argument functions
- Incremental garbage collection
- Garbage collection fixes
- Other misc bugs fixed
