Stack.XtExplicit transaction log passing on stacks.
swap s1 s2 exchanges the contents of the stacks s1 and s2.
to_seq s returns a domain safe sequence for iterating through the elements of the stack top to bottom.
The sequence is based on a constant time, O(1), snapshot of the stack and modifications of the stack have no effect on the sequence.
push x s adds the element x to the top of the stack s.
pop_opt s removes and returns the topmost element of the stack s, or None if the stack is empty.