Pops the data stack, returning the value.

def pop(self):
        """Pops the data stack, returning the value."""
        try:
            return self.data_stack.pop()
        except errors.MachineError as e:
            raise errors.MachineError("%s: At index %d in code: %s" %
                    (e, self.instruction_pointer, self.code_string))