?. ). Holds, From (18) and Lemma 4.3.13, there exists Q 3 such that (Q ? ) t 0 : (Q ? 3 , ?), We know from (16) that Q ? is already shared there exist Q ? 2 , ? 2 and ? ? such that (Q ? ) ?(t 0 ) : (Q ? 223) ?(Q 3 ) ? ? ? ? > ? ? Q ? 2 (24) ? ? ? ?(?) = ? 2

. Property, It is a consequence of the following

. Property, As a preliminary result, we show the same property for equivalence, that is, if (Q) ? 1 ? ? 2 holds, then ?(? 1 ) and ?(? 2 ) are equivalent in ML. The proof is by induction on the derivation of (Q) ? 1 ? ? 2 . Transitivity is by induction hypothesis. Reflexivity and symmetry are immediate: the ML equivalence relation is reflexive and symmetric. For congruence, the proof is similar to the one for the instance relation

@. Case, The premise is (Q) ? 1 ? ? ? 2 . Necessarily, we have (Q) ? 1 ? ? 2 (Rule eAbs-Equiv), because other possible rules for abstraction mention rigid bindings. Then, we conclude using the preliminary result

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