. We define
as the number of selectors from the class consisting of these
classes. Or we may equally well define
as the number of selections, for, since our classes of couples are mutually exclusive, the number of selectors is the same as the number of selections. A selection from our class of classes will be a set of ordered couples, of which there will be exactly one having any given member of
for its second term, and the first term may be any member of
