theorem
  [!f,x,x+h!] = (fD(f,h).x)/h
proof
  [!f,x,x+h!] = (-(f.(x+h)-f.x))/(-h) .= (f.(x+h)-f.x)/h by XCMPLX_1:191
    .= (fD(f,h).x)/h by DIFF_1:3;
  hence thesis;
end;
