Hint Rewrite
@field_at_data_at_cancel'
@field_at_data_at
@field_at__data_at_
@data_at__data_at : cancel.
(* END new experiments *)

Lemma change_compspecs_field_at_cancel3:
forall {cs1 cs2: compspecs} {CCE : change_composite_env cs1 cs2}
(sh: share) (t1 t2: type) gfs
(v1: @reptype cs1 (@nested_field_type cs1 t1 gfs))
(p: val),
t1 = t2 ->
cs_preserve_type cs1 cs2 (@coeq cs1 cs2 CCE) t1 = true ->
@field_at cs1 sh t1 gfs v1 p |-- @field_at_ cs2 sh t2 gfs p.