Hint Resolve legal_nested_field0_field.

Lemma nested_field_array_offset_in_range: forall t gfs lo hi,
legal_nested_field0 t (ArraySubsc lo :: gfs) ->
legal_nested_field0 t (ArraySubsc hi :: gfs) ->
complete_legal_cosu_type t = true ->
0 <= nested_field_offset t (ArraySubsc lo :: gfs) /\
nested_field_offset t (ArraySubsc lo :: gfs) + sizeof (nested_field_array_type t gfs lo hi) <= sizeof t.