Dead code.
Preserve compatibility on examples such as "intros []." on goals such
as "forall x:nat*nat, x=x", which resulted in
"forall n n0 : nat, (n, n0) = (n, n0)" before commit
37f68259ab0a33c3b5b41de70b08422d9bcd3bec on "Fixing introduction
patterns * and ** ".