theorem Th4:
  bD(f,h).x = f.x - f.(x-h)
proof
  thus bD(f,h).x = (-fD(f,-h)).x by RFUNCT_1:19
    .= - fD(f,-h).x by VALUED_1:8
    .= - (f.(x+-h) - f.x) by Th3
    .= f.x - f.(x-h);
end;
