Iterator.Step
enum Step[a: Type]Sourcecase Ans(a)case Skipcase DoneRepresent the "steps" of an iterator.
This is an expanded version of the Option type.
Ans is the answer at an intermediate stage of iteration.
Done signals that iteration is complete.
Skip is essentially an implementation detail. It allows us
to define filter and similar functions that "wither" Iterators
without recursively running through the Iterator.