theorem
  fD(cD(f,h),h).x = fD(f,h).(x+h/2)-cD(f,h).x
proof
  fD(cD(f,h),h).x = cD(f,h).(x+h)-cD(f,h).x by DIFF_1:3
    .= (f.((x+h)+h/2)-f.((x+h)-h/2))-cD(f,h).x by DIFF_1:5
    .= f.((x+h/2)+h)-f.(x+h/2)-cD(f,h).x
    .= fD(f,h).(x+h/2)-cD(f,h).x by DIFF_1:3;
  hence thesis;
end;
