Owl_utils_stackSourceType of a stack.
``pop stack`` pops the top element in ``stack``. It returns ``None`` if the ``stack`` is empty.
``peek stack`` returns the value of top element in ``stack`` but it does not remove the element from the stack. ``None`` is returned if the stack is empty.
``mem stack x`` checks whether ``x`` exist in ``stack``. The complexity is ``O(n)`` where ``n`` is the size of the ``stack``.
Similar to ``mem`` but physical equality is used for comparing values.