Yes I understand these techniques, in fact they are heavily in use in Colm, just not for maintaining the global state that is used for the parsing feedback loop

Yes I understand these techniques, in fact they are heavily in use in Colm, just not for maintaining the global state that is used for the parsing feedback loop. I use the term "copy" in the general sense to make discussion easier. Conceptually, persistent data structures are a copy, even if they are optimized heavily to incur very little cost.