End Partition.

Lemma Partition_fold :
forall (A:Type)(eqA:A->A->Prop)(st:Equivalence eqA)(f:key->elt->A->A),
Proper (E.eq==>eq==>eqA==>eqA) f ->
transpose_neqkey eqA f ->
forall m m1 m2 i,
Partition m m1 m2 ->
eqA (fold f m i) (fold f m1 (fold f m2 i)).