Description: Definition of the Morse couple. See df-bj-1upl . 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-Oct-2018)(New usage is discouraged.)