def phi_max(self, mechanism):
        """Return the |small_phi_max| of a mechanism.

        This is the maximum of |small_phi| taken over all possible purviews.
        """
        return min(self.mic(mechanism).phi, self.mie(mechanism).phi)
Return the |small_phi_max| of a mechanism.

        This is the maximum of |small_phi| taken over all possible purviews.