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 visitdir(self, dir):
        m1dir = self.m1.visitdir(dir)
        m2dir = self.m2.visitdir(dir)

        # if both matchers return "all" then we know for sure we don't need
        # to visit this directory. Same if all matchers return False. In all
        # other case we have to visit a directory.
        if m1dir == "all" and m2dir == "all":
            return False
        if not m1dir and not m2dir:
            return False
        return True