fun merge(a, []) = a
  | merge([], b) = b
  | merge(x::restx, y::resty) = 
    if x < y 
    then x::merge(restx, y::resty)
    else y::merge(x::restx, resty);


(* NOTE: The following will *not* work *)

fun merge(a, []) = a
  | merge([], b) = b
  | merge(x::restx, y::resty) = 
    if x < y 
    then x::y::merge(restx, resty)
    else y::x::merge(restx, resty);