Registers a configuration.

            Returns True if the configuration has been added and False if
            it already exists. Reports an error if the configuration is 'used'.

def register(self, id):
        """
            Registers a configuration.

            Returns True if the configuration has been added and False if
            it already exists. Reports an error if the configuration is 'used'.
        """
        assert isinstance(id, basestring)
        if id in self.used_:
            #FIXME
            errors.error("common: the configuration '$(id)' is in use")

        if id not in self.all_:
            self.all_.add(id)

            # Indicate that a new configuration has been added.
            return True
        else:
            return False