def add(self, child):
if self._child is not None:<EOL><INDENT>self._detach_child(self._child)<EOL><DEDENT>self._child = self._attach_child(child)<EOL>assert len(self) == <NUM_LIT:1><EOL>self._repack_and_regroup_children()<EOL>
Put a widget into the bin.

If the bin already had a child, it is removed and replaced by the new 
one.