Definition pointer_range_overlap p n p' n' :=
  exists l l', val2adr p l /\ val2adr p' l' /\ range_overlap l n l' n'.

Lemma pointer_range_overlap_dec: forall p1 n1 p2 n2, {pointer_range_overlap p1 n1 p2 n2} + {~ pointer_range_overlap p1 n1 p2 n2}.