Description: Define the infimum of class A . It is meaningful when R is a
relation that strictly orders B and when the infimum exists. For
example, R could be 'less than', B could be the set of real
numbers, and A could be the set of all positive reals; in this case
the infimum is 0. The infimum is defined as the supremum using the
converse ordering relation. In the given example, 0 is the supremum of
all reals (greatest real number) for which all positive reals are greater.
(Contributed by AV, 2-Sep-2020)