// finds lowest index where predicate turns true.
// returns "to" when predicate is false on whole range.
// Predicate operates on a range and returns true
// iff any value in the range is contained in the set.
static <A> int indexOfRangePredicate(int from default 0, int to, IF2<Int, Int, Bool> pred) {
  // interval empty?
  if (from >= to) ret to;
  
  // predicate false on whole range?
  if (!pred.get(from, to)) ret to;
  
  while ping (to-from > 1) {
    // predicate turns true somewhere in range. split in middle.
    int mid = (from + to) >>> 1;
    
    // check lower half
    if (pred.get(from, mid))
      // predicate turns true in lower half. move there.
      to = mid;
    else
      // lower half is clean, move to higher half
      from = mid;
  }

  // we've narrowed things down to at most one element, return index
  ret from;
}

Began life as a copy of #1029232