  For any inelastic process $v_{\ell} + I \to \ell^- + F$ with $m_{\ell} = 0$,
the cross section at $\theta_{\ell} = 0$ is given by Adler's PCAC theorem.
Inclusion of the lepton mass has a dynamical effect (``PCAC-screening'') caused
by interference of spin-zero ($\pi^+$) and spin-one exchanges. This effect may
be relevant to the forward suppression reported in recent experiments.
