<INTERFACE> The <CD/ORD_KEY/ signature <SEEALSO> <SIGREF/ORD_MAP/ <SIGREF/ORD_SET/ <FCTREF/BinarySetFn/ <FCTREF/BinaryMapFn/ <FCTREF/SplaySetFn/ <FCTREF/SplayMapFn/ <FCTREF/ListSetFn/ <FCTREF/ListMapFn/ </SEEALSO> <PP> The <SIGREF NOLINK/ORD_KEY/ signature provides an abstract description of ordered keys. It specifies a type plus a trivalent comparison function. <SPEC> <TYPE><ID>ord_key <SPEC> <VAL>compare<TY>(ord_key * ord_key) -> order <COMMENT> <PROTOTY> compare (<ARG/key/, <ARG/key'/) </PROTOTY> returns </INTERFACE>