Hint Resolve residuals_lift.

Lemma residuals_subst_rec :
forall U1 U2 U3 V1 V2 V3 : redexes,
residuals U1 U2 U3 ->
residuals V1 V2 V3 ->
forall k : nat,
residuals (subst_rec_r U1 V1 k) (subst_rec_r U2 V2 k) (subst_rec_r U3 V3 k).