Adds the functionality of the other CLI to this one, where all
        commands to the other CLI are prefixed by the given prefix plus a hyphen.

        e.g. To execute command `greet anson 5` on other cli given prefix cli2, you
        can execute the following with this CLI:
            cli2 greet anson 5

def add_cli(self, prefix, other_cli):
        """Adds the functionality of the other CLI to this one, where all
        commands to the other CLI are prefixed by the given prefix plus a hyphen.

        e.g. To execute command `greet anson 5` on other cli given prefix cli2, you
        can execute the following with this CLI:
            cli2 greet anson 5

        """
        if prefix not in self.clis and prefix not in self.cmds:
            self.clis[prefix] = other_cli
        else:
            raise ValueError('Attempting to overwrite cmd or extern CLI: %s' % prefix)