def insert(self, widget, layer):
self._attach_child(widget)<EOL>self._children[widget] = layer<EOL>self._repack_and_regroup_children()<EOL>
Insert a widget into the given layer of the stack.

Smaller layer numbers (e.g. 0) will be displayed behind larger ones 
(e.g. 1), the same as with `pyglet.graphics.OrderedGroup`.  Note that 
this method does not update the layer of any other widget besides the 
one being inserted, and it is possible for multiple widgets to occupy 
the same layer.

See `add()` for more details.