Sorry, I wasn't clear.  I wasn't suggesting it should be possible to change it, but what you've written here seems to suggest that it can be changed by the user "base of log  is e by default but **can be changed as per need**".  If the user has to modify the library source code, I wouldn't describe that as something which can be changed (pretty much everything can be changed if you allow for that).

Changing the base of the log just scales the weights by a constant factor, so it isn't worth the complication to the code and API, but the documentation really should match what the code does.

I think doxygen supports <sub>...</sub> for subscript text, so I'd suggest just saying (or if <sub> doesn't work here, ```ln(tf)```):

```
'L': Logarithmic tfn=1+log<sub>e</sub>(tf)
```

And leave it at that.