event log

> 0   (sequence (only-tags (sequence (capture 1 :a) (capture 2 :b))) (backref :a))
> 1   (only-tags (sequence (capture 1 :a) (capture 2 :b)))
> 2   (sequence (capture 1 :a) (capture 2 :b))
> 3   (capture 1 :a)
> 4   1
  4 > 1
  3 > (capture 1 :a)
> 5   (capture 2 :b)
> 6   2
  6 > 2
  5 > (capture 2 :b)
  2 > (sequence (capture 1 :a) (capture 2 :b))
  1 > (only-tags (sequence (capture 1 :a) (capture 2 :b)))
> 7   (backref :a)
  7 > (backref :a)
  0 > (sequence (only-tags (sequence (capture 1 :a) (capture 2 :b))) (backref :a))