AABB to AABB collision time function

Thanks Merx and Human. I'll get to it. I have now managed to find an AABB to AABB collision function in a book (that coincidentally just arrived from Amazon today!). I haven't had much time to look throught it yet. All the example code is in C but I should be able to convert it to BlitzMax (though my knowlede of C isn't great). It looks like a really good book for polygonal collision detection and determining collision time. Its called Real Time Collision Detection by Christer Ericson. Everyone loves it on the net - which is why I ordered it (though I wasn't really sure exactly what was in it until now).
Before I can do much though I need to know how to change variables that you pass to a function - inside that function. Without using Global Variables

do e.g

x=1;y=2
change(x,y)
'hopefully now x=3;y=5

Function change(a:int,b:int)
       a=a+2;b=b+2
end function
 

I know in C you put an & sign before the variable in the function definition.

Thanks - Rico