Notation val_inject:= Val.inject.

Theorem drop_extends:
forall m1 m2 lo hi b p m1',
Mem.extends m1 m2 ->
Mem.drop_perm m1 b lo hi p = Some m1'  ->
exists m2',
Mem.drop_perm m2 b lo hi p = Some m2'
/\ Mem.extends m1' m2'.