'Works like :meth:`set` but does not overwrite the values of already
existing keys.
:param key: the key to set
:param value: the value for the key
:param timeout: the cache timeout for the key in seconds (if not
specified, it uses the default timeout). A timeout of
0 idicates that the cache never expires.
:returns: Same as :meth:`set`, but also ``False`` for already
existing keys.
:rtype: boolean'

def add(self, key, value, timeout=None):

    return True
