Hint Resolve residuals_subst_rec.
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(* The Commutation Theorem *)
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Theorem commutation :
forall U1 U2 U3 V1 V2 V3 : redexes,
residuals U1 U2 U3 ->
residuals V1 V2 V3 ->
residuals (subst_r V1 U1) (subst_r V2 U2) (subst_r V3 U3).