theorem
  fD(bD(f,h),h).x = fD(f,h).x-bD(f,h).x
proof
  fD(bD(f,h),h).x = bD(f,h).(x+h)-bD(f,h).x by DIFF_1:3
    .= (f.(x+h)-f.((x+h)-h))-bD(f,h).x by DIFF_1:4
    .= fD(f,h).x-bD(f,h).x by DIFF_1:3;
  hence thesis;
end;
