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 _getN(self, n):
        n -= 1
        p = self.head
        while n:
            p = p.next
            n -= 1
        return p