The equation in documentation and the source code is not consistent

Hello,

I am trying to use a viscoelastic contact model to do the simulation. In the documentation, it shows there are 4 options to assign the parameters and I choose to used the 3rd option, where kn1, ks1, kn2, ks2 are assigned. The documentation shows that the equivalent normal stiffness is kn=2*kn1*kn2/(kn1+kn2), as shown in:

https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.Law2_ScGeom_ViscElPhys_Basic
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If normal and tangential stiffnesses (kn, ks) and damping constant (cn,cs) of each particle are precised, the equivalent stiffnesses and damping constants of each contact made of two particles 1 and 2 is made A=2*a1*a2/(a1+a2).
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However, I checked the source code of viscoelastic model and the kn in the source code is: kn=kn1*kn2/(kn1+kn2), the factor "2" is missing comparing to the documentation. The equation in the source code is shown in line 309 of https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/ViscoelasticPM.cpp

I am confused with this contradiction between the source code and the documentation. If you know which is the correct one, I wonder if you can help me, thank you!

Regards,
Yuxuan