let
get_a r =
let
(Pre x) = r.a
in
x;;
let
get_b r =
let
(Pre x) = r.b
in
x;;
let
with_a r x = { a = Pre x; b = r.b };;
let
with_b r x = { a = r.a; b = Pre x };;
let
vide = { a = Abs; b = Abs };;