reserve n,m for Element of NAT;
reserve h,k,r,r1,r2,x,x0,x1,x2,x3 for Real;
reserve f,f1,f2 for Function of REAL,REAL;

theorem
  fdif(f,h).1 = fD(f,h)
proof
  fdif(f,h).1 = fdif(f,h).(0+1) .= fD(fdif(f,h).0,h) by DIFF_1:def 6
    .= fD(f,h) by DIFF_1:def 6;
  hence thesis;
end;
