Definition precise (R : pred) : Prop := forall o m1 m2,
m1 |= R -> m2 |= R ->
subheap m1 o -> subheap m2 o -> eqmap m1 m2.

Lemma precise_FF : precise FF.
Definition mapsto x v : pred := fun m => forall n, m n = if eq_nat_dec x n then Some v else None.