Section UnpackStratifiedSemantics.
Context {W V G S' U F world_erase VU GF}
`{SS : StratifiedSemantics W V G S' U F world_erase VU GF}.

Lemma stratified_expr_safety : forall e e' w b1,
expr_erase GF e e'
-> exprEval G e w b1
-> exists b2, exprEval F e' (world_erase w) b2.