Thread-priodities

[Charma]

Hello,
i am currently working on a little programm which will use threads... I 
want to start two threads, one with the same priority as the programm 
itself and one with very low priority.
As i found in phobos, there is this setPriority() function but i can't 
input a number or somthing in there only INCREASE and DECREASE and so on...
Does anyone know how many steps there are from lowest to highest 
priority and how to use this function? if i want to lower the priority 
of one thread by two, do i need to run the function twice with decrease 
or will it end up the same?!
THanks for any help, Sorry for my engrish :)

Greeds,

Charma