[FLOW3-general] Where to put static tool class ?

[Dawid Pacholczyk]

Hi List,
  In my System I`d like to create a class with some special methods 
(tool methods, validating some specific things). For example tak 
t3lib_div in TYPO3 v4

Where should it land in FLOW3 architecture ? As for me it`s not a 
controller, model or repository.
So imo best way is to create a special lib folder with this class. Is it 
proper way ?

Best regards,
Dawid Pacholczyk