MOD function

Instead of using an API, you can simply do it (ignoring complications if the numbers were negative):

m = mod (a,b) can be computed as:
t = a / b    (where t is integer)
m = a - b * t

OR 

 >> Has anyone written a MOD() function? .......

It is called %REM in RPG IV - your example:

if mod(12/4) = 0

Can be coded as:

if %REM(12:4) = 0

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