 To prove the Lemma one shows that Id, MP and Cont, together with the “Curry-intersubstitutivity” of \(\kappa\) with \(\kappa {\rightarrow}\pi\), suffice to establish \(\vdash_{\mathcal{T}} \pi\). The following derivation resembles the informal argument of  section 1.1.  That argument also included a subargument for the principle Cont, which will be examined below.