Mark the decorated command as the intended entrypoint of the
    command module.

def entrypoint(cls):
    """Mark the decorated command as the intended entrypoint of the
    command module.

    """
    if not isinstance(cls, type) or not issubclass(cls, Command):
        raise TypeError(f"inappropriate entrypoint instance of type {cls.__class__}")
    cls._argcmdr_entrypoint_ = True
    return cls