# type graph = Graph of string * graph;;
type graph = Graph of string * graph
# let rec a = Graph("tak",b) and b = Graph("tak",a);;
val a : graph =
  Graph ("tak",
   Graph ("tak",
...
val b : graph =
  Graph ("tak",
   Graph ("tak",
...
# a == b;;
- : bool = false
# a = b;;
(* Ää»ß¤·¤Ê¤¤ *) 
# match a with c -> a == c;;
- : bool = true
(*************************************)
# let a = { x = "takashi" };;
val a : mutable_record = {x = "takashi"}
# match a with c -> begin c.x <- "mamewo"; a end;;
- : mutable_record = {x = "mamewo"}