Section STABILITY.
Variables env env': composite_env.
Hypothesis extends: forall id co, env!id = Some co -> env'!id = Some co.

Lemma Cop_sem_binary_operation_stable:
forall b v1 e1 v2 e2 m,
binop_stable env b e1 e2 = true ->
Cop.sem_binary_operation env b v1 (typeof e1) v2 (typeof e2) m =
Cop.sem_binary_operation env' b v1 (typeof e1) v2 (typeof e2) m.