theorem Th102:
  <*d1,d2,d3*> in 3-tuples_on D
proof
  <*d1,d2,d3*> in the set of all
 <*c1,c2,c3*> where c1 is (Element of D),c2 is (Element
  of D),c3 is Element of D ;
  hence thesis by Th100;
end;
