(* 日付けを表す型 *) 
type year_t = January of int 
            | February of int 
            | March of int 
            | April of int 
            | May of int 
            | June of int 
            | July of int 
            | August of int 
            | September of int 
            | October of int 
            | November of int 
            | December of int