theorem Th17:
  dom pr1(a,b) = a[x]b & cod pr1(a,b) = a
proof
  set p1 = (the Proj1 of C).(a,b), p2 = (the Proj2 of C).(a,b);
  a[x]b is_a_product_wrt p1,p2 by Def8;
  hence thesis by Def8;
end;
