
theorem
  for a,b being R_eal holds Sum<*a,b*> = a+b
proof
  let a,b be R_eal;
  thus Sum<*a,b*> = Sum(<*a*>^<*b*>) by FINSEQ_1:def 9
    .= Sum<*a*> + b by Lm1
    .= a + b by Th6;
end;
