code: function push(item, a)
  {
  	var pushed = push_(item, a);
  	if (pushed !== null)
  	{
  		return pushed;
  	}
  
  	var newTree = create(item, a.height);
  	return siblise(a, newTree);
  }
docs: Pushes an item via push_ to the bottom right of a tree.