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.