[TYPO3-mvc] own extension library

[Daniel Dimitrov]

Hey guys,
I'm wondering about the following thing. When we write extensions we often 
need to write functions that we use in several models.
Where is the best place to place those functions?
in Resources/Private/functions/... ?

Also what would be the best way to register those functions for use in my 
extension? I don't think that require(filename) everywhere I need a function 
is the best way to do this.

Kind Regards,
Daniel