question on on class emcee.moves.GaussianMove(cov, mode='vector', factor=None)

[Hui Tong]

Hi everyone, 

I am trying to use simple MH GaussianMove but I am a little confused that For class` emcee.moves.GaussianMove(cov, mode='vector', factor=None)`, the emcee docs (and the comments in the source code) all say the parameter `factor (Optional[float])` is invalid for the "vector" mode. However, I do not find factor option is forbidden in actual calculation of proposal samples when mode="vector" in the following code. Also, I don't quite understand mathematically why factor (Optional[float])` is invalid for the "vector" mode. 

Can someone point out where in the source code the factor option is disabled in vector mode, and theoretically why the factor parameter cannot be used in vector mode?

Thanks in advance!

Hui