A stack of expressions that can be used to support evaluation.
[... , a, b] => [..., cons(a,b)]
b must be cons or nil.
[... , (head, rest)] => [..., head]
[..., expr] => [...]
[...] => [..., expr]
[... , (head, rest)] => [..., rest]
Optional
[..., a, b] => [..., b, a]
A stack of expressions that can be used to support evaluation.