gistfile1.clj

(def end (p/not-followed-by p/anything))

(def lang1
  (p/alt
    (p/invisi-conc (term (partial = :a)) end)
    (p/invisi-conc (term (partial = :a)) (term (partial = :a)) end)))

(def lang2
  (p/invisi-conc (p/alt (term (partial = :a))
                        (p/invisi-conc (term (partial = :a)) (term (partial = :a))))
                 end))

(lang1 {:remainder [:a :a]}) 
; => [:a {:remainder nil}]

(lang2 {:remainder [:a :a]})
; => nil