Rightward drift in deep function application

[shwestrick]

This output has a too much rightward drift in the first branch:

fun filterIdx p s =
  case s of
    Flat (Full slice) =>
      Flat
        (Full
           (AS.full
              (SeqBasis.filter gran (0, AS.length slice) (AS.nth slice)
                 (fn k => p (k, AS.nth slice k)))))

  | Flat (Delay (i, j, f)) =>
      Flat (Full (AS.full (SeqBasis.filter gran (i, j) f (fn k => p (k, f k)))))

  | _ => filterIdx p (force s)

If we increase the max width to 100, it becomes more reasonable again:

fun filterIdx p s =
  case s of
    Flat (Full slice) =>
      Flat (Full (AS.full (SeqBasis.filter gran (0, AS.length slice) (AS.nth slice) (fn k =>
        p (k, AS.nth slice k)))))

  | Flat (Delay (i, j, f)) =>
      Flat (Full (AS.full (SeqBasis.filter gran (i, j) f (fn k => p (k, f k)))))

  | _ => filterIdx p (force s)

To fix this, I think "splittable expressions" need to be generalized to handle multiple possible splits...