Description: Define the value of an operation. In contrast to df-ov , the
alternative definition for a function value (see df-afv ) is used. By
this, the value of the operation applied to two arguments is the universal
class if the operation is not defined for these two arguments. There are
still no restrictions of any kind on what those class expressions may be,
although only certain kinds of class expressions - a binary operation
F and its arguments A and B - will be useful for proving
meaningful theorems. (Contributed by Alexander van der Vekens, 26-May-2017)