// 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;

}