theorem Th48:
  F is RAT-valued implies Sum F = addrat "**" F
proof
  assume A1:F is RAT-valued;
  rng F c= COMPLEX by A1,MEMBERED:1;
  then A2:F is COMPLEX-valued by RELAT_1:def 19;
  per cases by NAT_1:14;
    suppose A3:len F=0;
      hence addrat "**" F = 0 by Def8,A1,BINOP_2:3
                          .= Sum F by Def8,A2,A3,BINOP_2:1;
    end;
    suppose A4:len F>=1;
         A5: RAT = RAT /\ COMPLEX by MEMBERED:1,XBOOLE_1:28;
  now let x,y;assume x in RAT & y in RAT;
  then reconsider X=x,Y=y as Element of RAT;
   addrat.(x,y) = X+Y by BINOP_2:def 15;
   hence addrat.(x,y) =addcomplex.(x,y) & addrat.(x,y) in RAT
     by BINOP_2:def 3,RAT_1:def 2;
 end;
hence thesis by Th46,A4,A5,A1;
    end;
end;
