SML Function Argument Pattern Matching

I am using pattern matching on argument passed to a function. The method works fine for "first level" matching so to say but any attempt to go deeper gives the error "stdIn:282.5-291.77 Error: match redundant"

example

fun nnf T = T
    | nnf F = F
    | nnf (LETTER(x)) = (LETTER(x))
    | nnf (NEG(x)) = (NEG(nnf x))
    | nnf (AND(x,y)) = (AND(nnf x, nnf y))
    | nnf (OR(x,y)) = (OR(nnf x, nnf y))
    | nnf (IMP(x,y)) = (OR(NEG(nnf x),(nnf y)))
    | nnf (NEG(NEG(LETTER(x)))) = (LETTER(x))
    | nnf (NEG(AND(LETTER(x),LETTER(y)))) = (OR(NEG(LETTER(x)),NEG(LETTER(y))))
    | nnf (NEG(OR(LETTER(x),LETTER(y)))) = (AND(NEG(LETTER(x)),NEG(LETTER(y))));
val nnf = fn : prop -> prop

the error i get is

stdIn:282.5-291.77 Error: match redundant
          T => ...
          F => ...
          LETTER x => ...
          NEG x => ...
          AND (x,y) => ...
          OR (x,y) => ...
          IMP (x,y) => ...
    -->   NEG (NEG (LETTER x)) => ...
    -->   NEG (AND (LETTER x,LETTER y)) => ...
    -->   NEG (OR (LETTER x,LETTER y)) => ...

so SML is saying that the last 3 clauses in the function definition are the same since they all begin with "(NEG(..." even though what follows is different.

how do i overcome this ?

Solution

The case NEG(x) already covers any possible case involving outermost NEG, so the others are never reached. Depending on what you actually want to achieve, either remove that case, or move it after the more specific ones (cases are tried in order).