theorem Th2:
  fD(f,-h/2).x = -bD(f,h/2).x
proof
  fD(f,-h/2).x = f.(x-h/2) - f.x by DIFF_1:3
    .= -(f.x - f.(x-h/2))
    .= -bD(f,h/2).x by DIFF_1:4;
  hence thesis;
end;
