If you can try to keep line lengths < 80, things view better online (and in my editor).

You can stack up template parameters below each other (lined up on the left--- not sure if my counting chars here in this non-proportional font for the editor).  And you can start the variable on a new line.

```
DoubleVectorView<include_summand<propto,T_scale>::value,
                 is_vector<T_scale>::value> 
log_sigma(length(sigma));