formula used for pair_style gran/hooke/history for computation

Dear Lammps users,
The elastic term in normal force term in gran style for the calculation of frictional force between two granular particles is given in the Lammps documentation without positive sign. According to the sign convention given in Cundall , normal force acts in the direction opposite to the normal vector of the particle (Fig 3). But here the negative sign is omitted. i am not clear why negative sign is not given for the elastic normal force term.

These are the eqs in the Silbert paper, cited at the bottom.

You can see it defines Nij as Ni - Nj, same as Vij, so that implicitly includes

the negative sign you are thinking of.