Restore the heap property for a heap with an out of place element
    at position ``elt``. This works with a heap pair where heap1 carries
    the weights and heap2 holds the corresponding elements.

def siftdown(heap1, heap2, elt):
    """Restore the heap property for a heap with an out of place element
    at position ``elt``. This works with a heap pair where heap1 carries
    the weights and heap2 holds the corresponding elements."""
    while elt * 2 + 1 < heap1.shape[0]:
        left_child = elt * 2 + 1
        right_child = left_child + 1
        swap = elt

        if heap1[swap] < heap1[left_child]:
            swap = left_child

        if right_child < heap1.shape[0] and heap1[swap] < heap1[right_child]:
            swap = right_child

        if swap == elt:
            break
        else:
            heap1[elt], heap1[swap] = heap1[swap], heap1[elt]
            heap2[elt], heap2[swap] = heap2[swap], heap2[elt]
            elt = swap