theorem
  Sum<*r1,r2,r3,r4*> = r1 + r2 + r3 + r4
proof
  thus Sum<*r1,r2,r3,r4*> = Sum<*r1,r2,r3*> + r4 by Th74
    .= r1 + r2 + r3 + r4 by Th78;
end;
