custom right-hand fingering

[Federico Bruni]

Hi all

I'd like to create a custom right-hand fingering. Where should I look 
at?
I've tried looking at the definitions of finger and rightHandFinger in 
music-functions-init.ly, but I don't know how to create a new one.

I'm transcribing a piece from a book which uses this notation to 
specify tapping and the right-hand finger used to produce it:

Tm = tapping with middle finger
Ti = tapping with index finger

Thanks in advance
Federico