Initialize constants using literal structs.
Allow symbol name to start with -.
Remove duplicate code.
Remove DEFINE5 macro.
Simplify length().
Simplify read_list().
Remove TSPECIAL type.
Fix indentation.
Terminate env list with Nil instead of null pointer.
Use macro for repetitive switch cases.
Fix error message.
Add setcar.
Add minus operator.
Add < operator.
Cleanup.
Allow _ in symbol name.
Add gensym.