Description: If a ring has only one element, it is the zero ring. According to
Wikipedia ("Zero ring", 14-Apr-2019,
https://en.wikipedia.org/wiki/Zero_ring): "The zero ring, denoted {0}
or simply 0, consists of the one-element set {0} with the operations +
and * defined so that 0 + 0 = 0 and 0 * 0 = 0.". (Contributed by AV, 14-Apr-2019)