def remove(self, elem):
        """Remove a requirement.

        :param elem: a string or :class:`Requirement` instance.
        """
        if isinstance(elem, text_type):
            for requirement in self:
                if requirement.package == elem:
                    return super(Requirements, self).remove(requirement)
        return super(Requirements, self).remove(elem)
def merge(left, right):
    if left is None:
        return right
    if right is None:
        return left
    if left.prio > right.prio:
        left.right = merge(left.right, right)
        left.update()
        return left
    else:
        right.left = merge(left, right.left)
        right.update()
        return right