parsing utilities
- threaded, lockstep vm
- limited support for direct / indirect left recursion rules
https://godoc.org/github.com/reusee/pav
- vm approach: Russ Cox, "Regular Expression Matching: the Virtual Machine Approach", https://swtch.com/~rsc/regexp/regexp2.html
- bounded left recursion: Sérgio Medeiros, Fabio Mascarenhas, Roberto Ierusalimsch, "Left Recursion in Parsing Expression Grammar", https://arxiv.org/abs/1207.0443