def _decorate(fn, key=None, value=None):
    """Add the garcon property to the function.

    Args:
        fn (callable): The function to alter.
        key (string): The key to set (optional.)
        value (any): The value to set (optional.)
    """
    if not hasattr(fn, '__garcon__'):
        setattr(fn, '__garcon__', dict())
    if key:
        fn.__garcon__.update({key: value})
