adamc@826: fun mapX [a] [b] [ctx ::: {Unit}] f =
adamc@801:     let
adamc@801:         fun mapX' ls1 ls2 =
adamc@801:             case (ls1, ls2) of
adamc@801:                 ([], []) => <xml/>
adamc@801:               | (x1 :: ls1, x2 :: ls2) => <xml>{f x1 x2}{mapX' ls1 ls2}</xml>
adamc@801:               | _ => error <xml>ListPair.mapX: Unequal list lengths</xml>
adamc@801:     in
adamc@801:         mapX'
adamc@801:     end