mod, modulo and % under 2.4 and 2.5

[W. eWatson]

About a year ago, I wrote a program that used mod() for modulo under 
2.5. Apparently, % is also acceptable, but the program works quite well. 
I turned the program over to someone who is using 2.4, and apparently 
2.4 knows nothing about mod(). Out of curiosity, what library is 
mod(a,b)(two args) in? It doesn't seem to be in numpy. It seems to be 
built-in. If so, why isn't it both 2.4 and 2.5?