Help sml - troubled

How to declare a function so Listn : ' ' a list -> ' ' a list -> bool, listn xs and ys return true.  

Example: lisen [#"1" #"2"]开发者_如何学编程 , [#"1" "#3"] return false and  [#"1" , #"2"] [#"2" , #"1"] return true

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Try this:

infix member
fun x member []         = false
  | x member (y::ys)    = x = y orelse x member ys;

fun listn (x::xs) ys = x member ys andalso listn xs ys
  | listn [] _       = true;