set x = the Element of X;
set f = (3 -tuples_on X) --> the Element of X;
take (3 -tuples_on X) --> the Element of X ; :: thesis: (3 -tuples_on X) --> the Element of X is 3 -ary
thus arity ((3 -tuples_on X) --> the Element of X) = 3 by Th2; :: according to COMPUT_1:def 21 :: thesis: verum