docs: 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)