Removes and returns the top element from the stack. A null is treated as an empty stack, so it raises the same a real empty stack would, not a null-reference error. When the stack is empty, including when is null. The stack to pop from.

Pushes an item onto the top of the stack. is the mutation target: unlike the read-only helpers in this module, a null has no empty instance to push onto, so it is a programmer error. When is null. The item to push. The stack to push onto.

Returns a new stack with elements in reverse order. A null is treated as empty and yields a new empty stack. The stack to reverse.

Returns the top element as Some, or None if the stack is empty or null. The stack to peek into.