Well, as you see this is inherently taking the spirit of GLL/GLR parser -- defer parse until we have all the information. The academic solution to this is not to do it on token level but introduce a parse tree that is "forkable", meaning a new persistent data structure is needed to "compress" the tree when we have different routes, and that thing is called: graph structured stack (https://en.wikipedia.org/wiki/Graph-structured_stack)