Even faster (typically) is
```
{ val lb = List.newBuilder[Symbol]; existentials.foreach(x => if (x ne null) lb += x); lb.result }
```
and faster still is
```
{ val lb = List.newBuilder[Symbol];
  var i = 0
  while (i< existentials.length) {
    val x = existentials(i)
    if (x ne null) b += x
    i += 1
  }
  b.result
}
```
How big of a bottleneck is this?  Is it worth going that low-level?