OK, I reckon that dodicat's solution is overly complicated and requires the user to do a fair amount of coding and dependent upon how many instances of the UDT we use in addition to understanding how it works.
On the other hand, fxm's solution is simplicity itself and works equally well for one or more instances of the UDT. It is described as a principle and seems to me can easily be expanded upon.
I was thinking of saving/loading state vectors of a PRNG.
However, the more I think about it the more I wonder of the benefit of saving/loading the state vectors in the first place. I don't think that I have many bad ideas, but I think that this may be one of them. So, I have decided to put it to one side unless a benefit sneaks up on me in the future; unless someone can think of a benefit.
Of course, fxm's solution may be useful for applications which have nothing to do with PRNGs.
Thanks, guys.