// finds highest index where predicate is true
// returns from-1 when predicate is false on whole range
// predicate operates on a range and returns true
// any value in range is contained
static <A> int indexOfRangePredicate(int from default 0, int to, IF2<Int, Int, Bool> pred) {
  // interval empty?
  if (from >= to) ret from-1;
  
  // predicate false on whole range?
  if (!pred.get(from, to)) ret from-1;
  
  do ping {
    // predicate is true somewhere in range. split in middle.
    int mid = (from + to) >>> 1;
    
    // check higher half
    if (mid < to && pred.get(mid, to)) {
      // predicate is true in higher half. move there, continue.
      
      // if higher half is only one element, we're done
      if (mid+1 == to) ret mid;
      
      from = mid;
      continue;
    }
  
    // higher half is clean, move to lower half
    to = mid;
  } while (from < to);

  // we've narrowed things down to a zero length range, return index
  ret from-1;
}