def require(self, *requirements):
needed = self.resolve(parse_requirements(requirements))<EOL>for dist in needed:<EOL><INDENT>self.add(dist)<EOL><DEDENT>return needed<EOL>
Ensure that distributions matching `requirements` are activated

        `requirements` must be a string or a (possibly-nested) sequence
        thereof, specifying the distributions and versions required.  The
        return value is a sequence of the distributions that needed to be
        activated to fulfill the requirements; all relevant distributions are
        included, even if they were already activated in this working set.