def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def split(root, minRight):
    if root is None:
        return None, None
    if root.key >= minRight:
        left, right = split(root.left, minRight)
        root.left = right
        root.update()
        return left, root
    else:
        left, right = split(root.right, minRight)
        root.right = left
        root.update()
        return root, right