def add(self, widget, size=None):
self.add_back(widget, size)<EOL>
Add the given widget to the layout.

The widget will be added to the back of the layout (i.e. right for 
`HBox`, bottom for `VBox`).  The `size` argument specifies how much 
space (i.e. width for `HBox`, height for `VBox`) to allocate for the 
"cell" that will contain the widget (if you think of the hbox/vbox as a 
1-dimensional grid).  The size can either be an integer number of 
pixels or the string `'expand'`:

- number of pixels (int): The specified number of pixels will be 
  allocated for the widget, unless that number is smaller than the 
  widget's minimum size (i.e. its claim).  In that case, the widget's 
  minimum size will be allocated instead (because a widget can't be 
  smaller than its minimum size).  For this reason, `size=0` is a 
  common setting meaning: "take as little space as possible".  Of 
  course, you can also specify `size=100` to make a cell exactly 100px 
  wide/tall, assuming that the widget in question is smaller than that.

- `'expand'` (str): A special value indicating that the widget should 
  expand to fill any space available to the container but not used by 
  any other cells.  For example, imagine you have a `HBox` that's 500px 
  wide (or a `VBox that's 500 px tall).  If you add two widgets with 
  `size='expand'`, each will get 250 px.  If you add one widget with `size=100` and two with `size='expand'`, the first will 

If no size is specified, a default is used.  The default can be set (in 
order of precedence) either via `set_default_cell_size()`, an argument 
to the constructor, or the `custom_default_cell_size` class variable.

Examples:

These are with `HBox`, but could equivalently be with `VBox`.  Assume 
for the sake of simplicity that the `HBox` is 500px wide.  Further 
assume that `w1`, `w2`, and `w3` are arbitrary widgets with no minimum 
size (e.g. `Placeholder`).

In this example, `w1` and `w2` will both be 250px wide (i.e. half the 
width of the container):

>>> h1 = glooey.HBox()  # 500px wide
>>> h1.add(w1, size='expand')
>>> h1.add(w2, size='expand')

In this example, `w1` will be 100px wide and `w2` and `w3` will split 
the remaining space and be 200px each:

>>> h2 = glooey.HBox()  # 500px wide
>>> h2.add(w1, size=100)
>>> h2.add(w2, size='expand')
>>> h2.add(w3, size='expand')