def add_command(self, command):
        """Adds a :class:`.Command` or its subclasses into the internal list
        of commands.

        This is usually not called, instead the :meth:`~.GroupMixin.command` or
        :meth:`~.GroupMixin.group` shortcut decorators are used instead.

        Parameters
        -----------
        command
            The command to add.

        Raises
        -------
        :exc:`.ClientException`
            If the command is already registered.
        TypeError
            If the command passed is not a subclass of :class:`.Command`.
        """

        if not isinstance(command, Command):
            raise TypeError('The command passed must be a subclass of Command')

        if isinstance(self, Command):
            command.parent = self

        if command.name in self.all_commands:
            raise discord.ClientException('Command {0.name} is already registered.'.format(command))

        self.all_commands[command.name] = command
        for alias in command.aliases:
            if alias in self.all_commands:
                raise discord.ClientException('The alias {} is already an existing command or alias.'.format(alias))
            self.all_commands[alias] = command
Adds a :class:`.Command` or its subclasses into the internal list
        of commands.

        This is usually not called, instead the :meth:`~.GroupMixin.command` or
        :meth:`~.GroupMixin.group` shortcut decorators are used instead.

        Parameters
        -----------
        command
            The command to add.

        Raises
        -------
        :exc:`.ClientException`
            If the command is already registered.
        TypeError
            If the command passed is not a subclass of :class:`.Command`.