def _setup(self, master, cnf):
    """Internal function. Sets up information about children."""
    if _support_default_root:
        global _default_root
        if master is None: # if not master:
            if not _default_root:
                _default_root = Tk()
            master = _default_root
    self.master = master
    self.tk = master.tk
    name = None
    if 'name' in cnf:
        name = cnf['name']
        del cnf['name']
    if not name:
        name = repr(id(self))
    self._name = name
    if master._w=='.':
        self._w = '.' + name
    else:
        self._w = master._w + '.' + name
    self.children = {}
    if self._name in self.master.children:
        self.master.children[self._name].destroy()
    self.master.children[self._name] = self