[Retros] A single integer N = a complete chess game

[Seth]

The concept of "most economical" is not well-defined in this space.
Is it "the maximum length for any possible game is the minimum"?  Is
it "the expected length (over what probability distribution?) is the
minimum"?  Must the strings be self-terminating (that is, can one game
be a prefix of another)?

Some form of "list all chess games in some order, and give the index
into that list" is likely best.  Now, which order?

Seth