I am extremely new to SML and we just got out first programming assignment for class and I need a little insight. The question is: write an ML function, called minus: int list * int list -> int list, that takes two non-decreasing integer lists and produces a non-decreasing integer list obtained by removing the elements from the first input list which are also found in the second input list. For example, minus( [1,1,1,2,2], [1,1,2,3] ) = [1,2] minus( [1,1,2,3],[1,1,1,2,2] ) = [3] Here is my attempt at answering the question. Can anyone tell me what I am doing incorrectly? I don't quite understand parsing lists. fun minus(xs,nil) = [] | minus(nil,ys) = [] | minus(x::xs,y::ys) = if x=y then minus(xs,ys) else x :: minus(x,ys); Here is a fix I just did, I think this is right now? fun minus(L1,nil) = L1 | minus(nil,L2) = [] | minus(L1,L2) = if hd(L1) > hd(L2) then minus(L1,tl(L2)) else if hd(L1) = hd(L2) then minus(tl(L1),tl(L2)) else hd(L1) :: minus(tl(L1), L2);