  We demonstrate that Robb-Geroch's definition of a relativistic interval
admits a simple and fairly natural generalization leading to a Finsler
extension of special relativity. Another justification for such an extension
goes back to the works of Lalan and Alway and, finally, was put on a solid
basis and systematically investigated by Bogoslovsky under the name
"Special-relativistic theory of locally anisotropic space-time". The isometry
group of this space-time, $\mathrm{DISIM}_b(2)$, is a deformation of the Cohen
and Glashow's very special relativity symmetry group $\mathrm{ISIM(2)}$. Thus,
the deformation parameter b can be regarded as an analog of the cosmological
constant characterizing the deformation of the Poincare group into the de
Sitter (anti-de Sitter) group. The simplicity and naturalness of Finslerian
extension in the context of this article adds weight to the argument that the
possibility of a nonzero value of $b$ should be carefully considered.
