Source code in the txr programming language

@(deffilter abbr
   ("IK" "I know an old lady who swallowed a") ("SW" "She swallowed the")
   ("SS" "she swallowed") ("CA" "to catch the") ("XX" "Perhaps she'll die")
   ("C" "cow") ("G" "goat") ("D" "dog") ("@" "cat") ("R" "bird")
   ("$" "spider") ("F" "fly"))
@(bind lastverse
   ("IK C"
    "I don't know how SS the C"
    "SW C CA G"
    "SW G CA D"
    "SW D CA @"
    "SW @ CA R"
    "SW R CA $"
    "SW $ CA F"
    "But I don't know why SS that F"
    "XX"
    ""
    "IK horse"
    "She's alive and well of course!"))
@(bind animal_line
  ("G: Opened her throat and down went the G!"
   "D: What a hog to swallow a D!"
   "@: Imagine that! She swallowed a @!"
   "R: How absurd to swallow a R!"
   "$: That wriggled and jiggled and tickled inside her"
   "F: But I don't know why SS the F"))
@(define expand_backwards (song lengthened_song done))
@  (local line2 line3 verse rest animal previous_animal previous_animal_verse)
@  (next :list song)
@  (cases)
IK @animal
@line2
SW @animal CA @previous_animal
@    (maybe)
But @(skip)F
@    (end)
@    (collect)
@    verse
@    (until)

@    (end)
@    (collect)
@    rest
@    (end)
@    (next :list animal_line)
@    (skip)
@previous_animal: @previous_animal_verse
@    (output :into lengthened_song)
IK @previous_animal
@previous_animal_verse
@      (repeat)
@      verse
@      (end)

@      (repeat)
@      song
@      (end)
@    (end)
@    (bind done nil)
@ (or)
IK @(skip)
@line2
XX
@    (bind lengthened_song song)
@    (bind done t)
@ (end)
@(end)
@(define expand_song (in out))
@  (local lengthened done)
@  (expand_backwards in lengthened done)
@  (cases)
@    (bind done nil)
@    (expand_song lengthened out)
@  (or)
@    (bind out lengthened)
@  (end)
@(end)
@(expand_song lastverse song)
@(output :filter abbr)
@  (repeat)
@song
@  (end)
@(end)