package owl-base

Type of a stack.

``make ()`` creates an empty stack.

``push stack x`` pushes ``x`` into ``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.

Returns ``true`` if the stack is empty, otherwise ``false``.

``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.

``to_array stack`` converts the elements in ``stack`` into an array.