theorem
  product <*D,D,D*> = 3-tuples_on D
proof
  thus product <*D,D,D*> = the set of all <*d1,d2,d3*>
  where d1 is Element of D, d2 is Element of D,
     d3 is Element of D by Th127
    .= 3-tuples_on D by FINSEQ_2:102;
end;
