reserve h,r,r1,r2,x0,x1,x2,x3,x4,x5,x,a,b,c,k for Real,
  f,f1,f2 for Function of REAL,REAL;

theorem Th1:
  [!f,x,x+h!] = (fdif(f,h).1.x)/h
proof
  [!f,x,x+h!] = [!f,x+h,x!] by DIFF_1:29
    .= (fD(f,h).x)/h by DIFF_1:3
    .= (fD(fdif(f,h).0,h).x)/h by DIFF_1:def 6
    .= (fdif(f,h).(0 qua Nat+1).x)/h by DIFF_1:def 6;
  hence thesis;
end;
