Rename struct var to struct identifier.
Move variable name into token_roots.
Add struct modifiable.
Add iterators to class pool<T>.
Add a routine to clear a pool.
Replace context of parameters with modifiable(index).
Rename parameter_regs to modifiable_regs.
Make modifiable_type also not WHNF.
Simplify code.
Don't return index_vars, or mark them as inputs.
Clean up code and note's a bit.
Update notes.
Remove some old comments.
Assert that index_var's never have a result.
Pass evaluate_changeable down to next level.
Add template for IO operation to set modifiable values.
Remove old notes for defunct push/enter evaluation model.
Print names for parameters.
Add new operations to struct OperationArgs.