Forgetting to wrap a string in a (string) combinator should produce helpful error message

[vkz]

(i/parser
 {:foo (cat (string "foo") "." (string "bar"))}
 :start :foo)

;; =>
;; IllegalArgumentException No matching clause:   instaparse.cfg/seq-nt (cfg.clj:236)
;; followed by unhelpful stack

You can easily spot an error in this tiny grammar: we forgot to wrap "." in (string "."). Imagine however trying to find a problem in 100+ lines grammar when all you get is No matching clause: and the last frame that points to some internal library function. I'm not sure if there's a general way to identify and report cases like this, but it might save someone else plenty of time and head-scratching.

[Engelberg]

This is a good idea. There are a few error reporting things like this I want to do, so I'll add this to the list.