theorem
  (a,b,b) --> (x,y,z) = (a,b) --> (x,z)
  proof
    thus (a,b,b) --> (x,y,z) = (a .--> x) +* ((b,b) -->(y,z)) by Th14
    .= (a,b) --> (x,z) by Th81;
  end;
