Description: Define a general binary relation. Note that the syntax is simply three
class symbols in a row. Definition 6.18 of TakeutiZaring p. 29
generalized to arbitrary classes. Class R often denotes a relation
such as " < " that compares two classes A and B , which might
be numbers such as 1 and 2 (see df-ltxr for the specific
definition of < ). As a wff, relations are true or false. For
example, ( R = { <. 2 , 6 >. , <. 3 , 9 >. } -> 3 R 9 ) ( ex-br ).
Often class R meets the Rel criteria to be defined in df-rel ,
and in particular R may be a function (see df-fun ). This
definition of relations is well-defined, although not very meaningful,
when classes A and/or B are proper classes (i.e. are not sets).
On the other hand, we often find uses for this definition when R is a
proper class (see for example iprc ). (Contributed by NM, 31-Dec-1993)