src/Pure/proofterm.ML

       | lift' _ _ (PAxm (s, prop, Ts)) =
           PAxm (s, prop, apsome' (same (map (incr_tvar inc))) Ts)
       | lift' _ _ _ = raise SAME
     and lifth' Us Ts prf = (lift' Us Ts prf handle SAME => prf);
 
-    val ps = map lift_all (Logic.strip_imp_prems (snd (Logic.strip_flexpairs prop)));
+    val ps = map lift_all (Logic.strip_imp_prems prop);
     val k = length ps;
 
     fun mk_app (b, (i, j, prf)) = 
           if b then (i-1, j, prf %% PBound i) else (i, j-1, prf %> Bound j);