| ListP ps => let
            fun ifExists(x) = case ex of
            List xs => if(List.length xs <> List.length ps) then NONE else case (xs, ps) of
                (h::nil, g::nil) => if(Option.isSome(match(h, g))) then match(h, g) else NONE
                |(h::t, g::r) => if(Option.isSome(match(h, g))) then Option.join(SOME(SOME(Option.valOf(match(h,g)) @ Option.valOf(match(List t, ListP r))))) else NONE


        in
            ifExists(ps)
        end