@@ 0,0 1,50 @@ 
+#!/usr/bin/env bash
+# -*- wisp -*-
+exec -a "$0" guile -L "$(dirname "$(realpath "$0")")" -x .w --language=wisp -e '(rare-bigrams)' -c '' "$@"
+;; !#
+
+define-module : rare-bigrams
+    . #:export : main
+
+import : srfi srfi-1
+         ice-9 rdelim
+
+define : usage . args
+    format (current-error-port) "~a <2-gram-file.txt>"
+
+define : main args
+    when : null? : cdr args
+           apply usage args
+           exit 1
+    let : : port : open-input-file : second args
+        define seen-map : make-hash-table 10000
+        define : select-canonical-bigram line
+            list->string
+              stable-sort
+                string->list
+                  string-downcase
+                      string-join
+                          cdr : string-split line #\space
+                          . " "
+                . char<?
+        define : unknown? bigram
+            not : hash-ref seen-map bigram
+        define simple-lowercase-german-regexp
+            make-regexp "^[abcdefghijklmnopqrstuvwxyzäöüß]+$"
+        define : simple-lowercase-german? bigram
+            regexp-exec simple-lowercase-german-regexp bigram
+        define sorted-bigrams
+            let loop : (line (read-line port)) (unique (list))
+                 if : eof-object? line
+                      reverse unique
+                      loop (read-line port)
+                          let : : bigram : select-canonical-bigram line
+                             if : unknown? bigram
+                                 begin
+                                     hash-set! seen-map bigram #t
+                                     if : simple-lowercase-german? bigram
+                                          cons bigram unique
+                                          . unique
+                                 . unique
+        map : λ(x) : format #t "~a\n" x
+            . sorted-bigrams