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

Lemma field_offset_stable: forall i id co ofs,
composite_env_consistent env ->
env ! i = Some co ->
field_offset env id (co_members co) = Errors.OK ofs ->
field_offset env' id (co_members co) = Errors.OK ofs.