for i := a + 1; i < b; i++ {
		for j := i; j > a && data.Less(j, j-1); j-- {
			data.Swap(j, j-1)
		}
	}
}

// siftDown_func implements the heap property on data[lo:hi].
// first is an offset into the array where the root of the heap lies.
func siftDown_func(data lessSwap, lo, hi, first int) {
	root := lo
	for {
		child := 2*root + 1
		if child >= hi {
			break
		}