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 _remainingpats(pat, prefix):
    """list of patterns with prefix stripped

    >>> _remainingpats("a/b/c", "")
    ['a/b/c']
    >>> _remainingpats("a/b/c", "a")
    ['b/c']
    >>> _remainingpats("a/b/c", "a/b")
    ['c']
    >>> _remainingpats("a/b/c", "a/b/c")
    []
    >>> _remainingpats("", "")
    []
    """
    if prefix:
        if prefix == pat:
            return []
        else:
            assert pat[len(prefix)] == "/"
            return [pat[len(prefix) + 1 :]]
    else:
        if pat:
            return [pat]
        else:
            return []