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(* miscellanous                                   *)
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Lemma assoc_add_hasmem_effect :
forall (l0 l1 : assoc) (k k' : K) (v v' : V),
~ assoc_hasmem l0 k v ->
assoc_hasmem l1 k v -> l1 = assoc_add l0 k' v' -> k' = k /\ v' = v.