Calling doc.replaceChild(new_child, old_child) with new_child and old_child being similar nodes in two different documents results in new_child to be removed from its document instead of old_child being removed from doc.

Example:
        new_child = get_element_x(doc_a)
        old_child = get_element_x(doc_b)
        parent = get_element_y(doc_b)
        parent.replaceChild(new_child, old_child)

=> new_child will be removed from doc_a. Instead old_child should be removed from doc_b.