Unique constant per instantiation

But the value returned by any function is constant, and is generated when the function is called, and my impure function worked without a problem in Modelsim (which is pretty good at LRM compliance). You cannot use a procedure because it doesnt return a value (and hence has to be put in a process/function/another procedure). as it is essentially just a wrapping up of code.

I have raised a support request with Altera about this, as a function return value is always constant. They will probably argue that they dont support shared variables or signals in packages and then just close the case!