'Add a new tab with the name given in tab_name.'

def add_tab(self, tab_name):

    if (not tab_name):
        raise InvalidNameError(("Invalid    Tab    name:    '%s'" % tab_name))
    if (tab_name in self._tab_names):
        raise AlreadyExistsError(("Tab    named    '%s'    already    exists" % tab_name))
    self._tab_names.append(tab_name)
    self._arrange_tabs()
