The Simplification of the Induction Conclusion (Step 10)
Subgoal *1/2' (IMPLIES (AND (CONSP A) (EQUAL (APP (APP (CDR A) B) C) (APP (CDR A) (APP B C))))(EQUAL (APP (APP (CDR A) B) C) (APP (CDR A) (APP B C)))).
Click on the link above to use the Induction Hypothesis (which is the second of the two hypotheses above and which is identical to the rewritten conclusion).