obj Sex =  Male or Female;
val Male = "male";
val Female = "female";

val s:Sex = "male";  (* This is fine, since s's secondary type is
			    "(the string \"male\") or (the string \"female\")"
			which means we can type-safely assign "male" to it. *)

val s':Sex = "x";    (* This is not fine, since "x" is not compat with the
			secondary spelling of s'. *)

val s'':Sex = 10;    (* This is obviously wrong. *)