Description: Definition of the Morse monuple (1-tuple). This is not useful per se, but
is used as a step towards the definition of couples (2-tuples, or ordered
pairs). The reason for "tagging" the set is so that an m-tuple and an
n-tuple be equal only when m = n. Note that with this definition, the
0-tuple is the empty set. New usage is discouraged because the precise
definition is generally unimportant compared to the characteristic
properties bj-2upleq , bj-2uplth , bj-2uplex , and the properties of
the projections (see df-bj-pr1 and df-bj-pr2 ). (Contributed by BJ, 6-Apr-2019)(New usage is discouraged.)