fix in sortcheck_app

This edit addresses the issue ucsd-progsys/liquidhaskell#66
where
```haskell
{-@ listElem :: y:a -> zs:[a] -> {v:Bool | (Prop(v) <=> Set_mem(y, (listElts(zs))))} @-}
listElem ::  a -> [a] -> Bool
listElem = undefined
```
produces a sortcheck error