Deletion

Arguably, so as to return as balanced of a tree as possible, a breadth_first traversal of this tree should turn out [7, 4, 8, 2, 6, 10, 9, 19, 13, 20, 11, 23] instead of [7, 4, 8, 2, 6, 10, 9, 11, 13, 20, 19, 23], where 19 has moved to where 15 once was. This would make the balance 3/-3 instead of 4/-4.