  < 1 >

  < 2 >
  (sequence
    (comp
      cat ; <3>
  < 4 >
  < 5 >
    (re-seq #"\d{10}" pi-book)))

(take 20 pi-seq)
;; (1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6)
 <3>
 (1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6)
  < 1 >

  < 2 >
  (sequence
    (comp
  < 4 >
  < 5 >
    (re-seq #"\d{10}" pi-book)))

(take 20 pi-seq)