diff --git a/footer_with_songinfo.ly b/footer_with_songinfo.ly
index 919e182..8d5fe53 100644
--- a/footer_with_songinfo.ly
+++ b/footer_with_songinfo.ly
@@ -1,13 +1,19 @@
 #(define-markup-command (print-songinfo layout props) ()
+  (define (songinfo-from songId key)
+    (let ((song (if (defined? 'SONG_DATA) (assoc-ref SONG_DATA songId) #f)))
+      (if song
+        (assoc-ref song key)
+        (ly:warning (ly:format "song with id ~a not found" songId)))))
+
   (define (format-author authorId)
-    (let ((author (if (defined? 'AUTHORS) (assq-ref AUTHORS authorId) #f)))
+    (let ((author (if (defined? 'AUTHOR_DATA) (assoc-ref AUTHOR_DATA authorId) #f)))
       (if author
         (markup
-          #:override (cons 'author:name (assq-ref author 'name))
-          #:override (cons 'author:trail_name (assq-ref author 'trail_name))
-          #:override (cons 'author:birth_year (assq-ref author 'birth_year))
-          #:override (cons 'author:death_year (assq-ref author 'death_year))
-          #:override (cons 'author:organization (assq-ref author 'organization))
+          #:override (cons 'author:name (assoc-ref author "name"))
+          #:override (cons 'author:trail_name (assoc-ref author "trail_name"))
+          #:override (cons 'author:birth_year (assoc-ref author "birth_year"))
+          #:override (cons 'author:death_year (assoc-ref author "death_year"))
+          #:override (cons 'author:organization (assoc-ref author "organization"))
           (ly:output-def-lookup layout 'authorMarkup))
         (ly:warning (ly:format "author with id ~a not found" authorId)))))
 
@@ -22,9 +28,10 @@
 
   (interpret-markup layout props
     (if (chain-assoc-get 'page:is-bookpart-last-page props #f)
-    (let* ((poetId (chain-assoc-get 'header:poetId props #f))
-           (composerId (chain-assoc-get 'header:composerId props #f))
-           (poet-and-composer-same (eq? poetId composerId)))
+    (let* ((songId (chain-assoc-get 'header:songId props #f))
+           (poetId (chain-assoc-get 'header:poetId props (if songId (songinfo-from songId "poet") #f)))
+           (composerId (chain-assoc-get 'header:composerId props (if songId (songinfo-from songId "composer") #f)))
+           (poet-and-composer-same (equal? poetId composerId)))
       (let (
             (blockwidth (* (chain-assoc-get 'header:songinfo-size-factor props 0.9) (ly:output-def-lookup layout 'line-width)))
             (infotext (chain-assoc-get 'header:songinfo props #f))
diff --git a/general_include.ly b/general_include.ly
index c9c03ea..b25d908 100644
--- a/general_include.ly
+++ b/general_include.ly
@@ -6,6 +6,8 @@
 
 #(define noStandaloneOutput (if (defined? 'noStandaloneOutput) noStandaloneOutput #f))
 
+#(load "json_parser.scm")
+#(use-modules (json parser))
 \include "basic_format_and_style_settings.ly"
 \include #(if (< (list-ref (ly:version) 1) 25) "legacy-lilypond-compatibility-pre-2.25.ly" "void.ly")
 \include "eps_file_from_song_dir.ly"
diff --git a/json_parser.scm b/json_parser.scm
new file mode 100644
index 0000000..8f71415
--- /dev/null
+++ b/json_parser.scm
@@ -0,0 +1,482 @@
+;;; (json parser) --- Guile JSON implementation.
+
+;; Copyright (C) 2013-2020 Aleix Conchillo Flaque <aconchillo@gmail.com>
+;;
+;; This file is part of guile-json.
+;;
+;; guile-json is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; guile-json is distributed in the hope that it will be useful, but
+;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+;; General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with guile-json. If not, see https://www.gnu.org/licenses/.
+
+;;; Commentary:
+
+;; JSON module for Guile
+
+;;; Code:
+
+(define-module (json parser)
+  #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 streams)
+  #:use-module (rnrs io ports)
+  #:export (json->scm
+            json-string->scm
+            json-seq->scm
+            json-seq-string->scm))
+
+;;
+;; Miscellaneuos helpers
+;;
+
+(define (json-exception port)
+  (throw 'json-invalid port))
+
+(define (digit? c)
+  (case c
+    ((#\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9) #t)
+    (else #f)))
+
+(define (whitespace? c)
+  (case c
+    ((#\sp #\ht #\lf #\cr) #t)
+    (else #f)))
+
+(define (control-char? ch)
+  (<= (char->integer ch) #x1F))
+
+(define (skip-whitespaces port)
+  (let ((ch (peek-char port)))
+    (cond
+     ((whitespace? ch)
+      (read-char port)
+      (skip-whitespaces port))
+     (else *unspecified*))))
+
+(define (expect-string port expected return)
+  (let loop ((n 0))
+    (cond
+     ;; All characters match.
+     ((= n (string-length expected)) return)
+     ;; Go to next characters.
+     ((eqv? (read-char port) (string-ref expected n))
+      (loop (+ n 1)))
+     ;; Anything else is an error.
+     (else (json-exception port)))))
+
+(define (expect-delimiter port delimiter)
+  (let ((ch (read-char port)))
+    (cond
+     ((not (eqv? ch delimiter)) (json-exception port))
+     ;; Unexpected EOF.
+     ((eof-object? ch) (json-exception port)))))
+
+(define (skip-record-separators port)
+  (when (eqv? #\rs (peek-char port))
+    (read-char port)
+    (skip-record-separators port)))
+
+;;
+;; Number parsing helpers
+;;
+
+(define (expect-digit port)
+  (let ((ch (peek-char port)))
+    (cond
+     ((not (digit? ch)) (json-exception port))
+     ;; Unexpected EOF.
+     ((eof-object? ch) (json-exception port)))))
+
+;; Read + or -, and return 1 or -1 respectively. If something different is
+;; found, return 1.
+(define (read-sign port)
+  (let ((ch (peek-char port)))
+    (cond
+     ((eqv? ch #\+)
+      (read-char port)
+      1)
+     ((eqv? ch #\-)
+      (read-char port)
+      -1)
+     (else 1))))
+
+(define (read-digit-value port)
+  (let ((ch (read-char port)))
+    (cond
+     ((eqv? ch #\0) 0)
+     ((eqv? ch #\1) 1)
+     ((eqv? ch #\2) 2)
+     ((eqv? ch #\3) 3)
+     ((eqv? ch #\4) 4)
+     ((eqv? ch #\5) 5)
+     ((eqv? ch #\6) 6)
+     ((eqv? ch #\7) 7)
+     ((eqv? ch #\8) 8)
+     ((eqv? ch #\9) 9)
+     (else (json-exception port)))))
+
+;; Read digits [0..9].
+(define (read-digits port)
+  (expect-digit port)
+  (let loop ((ch (peek-char port)) (number 0))
+    (cond
+     ((digit? ch)
+      (let ((value (read-digit-value port)))
+        (loop (peek-char port) (+ (* number 10) value))))
+     (else number))))
+
+(define (read-digits-fraction port)
+  (expect-digit port)
+  (let loop ((ch (peek-char port)) (number 0) (length 0))
+    (cond
+     ((digit? ch)
+      (let ((value (read-digit-value port)))
+        (loop (peek-char port) (+ (* number 10) value) (+ length 1))))
+     (else
+      (/ number (expt 10 length))))))
+
+(define (read-exponent port)
+  (let ((ch (peek-char port)))
+    (cond
+     ((or (eqv? ch #\e) (eqv? ch #\E))
+      (read-char port)
+      (let ((sign (read-sign port))
+            (digits (read-digits port)))
+        (if (<= digits 1000) ;; Some maximum exponent.
+            (expt 10 (* sign digits))
+            (json-exception port))))
+     (else 1))))
+
+(define (read-fraction port)
+  (let ((ch (peek-char port)))
+    (cond
+     ((eqv? ch #\.)
+      (read-char port)
+      (read-digits-fraction port))
+     (else 0))))
+
+(define (read-positive-number port)
+  (let* ((number
+          (let ((ch (peek-char port)))
+            (cond
+             ;; Numbers that start with 0 must be a fraction.
+             ((eqv? ch #\0)
+              (read-char port)
+              0)
+             ;; Otherwise read more digits.
+             (else (read-digits port)))))
+         (fraction (read-fraction port))
+         (exponent (read-exponent port))
+         (result (* (+ number fraction) exponent)))
+    (if (and (zero? fraction) (>= exponent 1))
+        result
+        (exact->inexact result))))
+
+(define (json-read-number port)
+  (let ((ch (peek-char port)))
+    (cond
+     ;; Negative numbers.
+     ((eqv? ch #\-)
+      (read-char port)
+      (expect-digit port)
+      (* -1 (read-positive-number port)))
+     ;; Positive numbers.
+     ((digit? ch)
+      (read-positive-number port))
+     ;; Anything else is an error.
+     (else (json-exception port)))))
+
+;;
+;; Object parsing helpers
+;;
+
+(define (read-pair port null ordered)
+  ;; Read key.
+  (let ((key (json-read-string port)))
+    (skip-whitespaces port)
+    (let ((ch (peek-char port)))
+      (cond
+       ;; Skip colon and read value.
+       ((eqv? ch #\:)
+        (read-char port)
+        (cons key (json-read port null ordered)))
+       ;; Anything other than colon is an error.
+       (else (json-exception port))))))
+
+(define (json-read-object port null ordered)
+  (expect-delimiter port #\{)
+  (let loop ((pairs '()) (added #t))
+    (skip-whitespaces port)
+    (let ((ch (peek-char port)))
+      (cond
+       ;; End of object.
+       ((eqv? ch #\})
+        (read-char port)
+        (cond
+         (added (if ordered (reverse! pairs) pairs))
+         (else (json-exception port))))
+       ;; Read one pair and continue.
+       ((eqv? ch #\")
+        (let ((pair (read-pair port null ordered)))
+          (loop (cons pair pairs) #t)))
+       ;; Skip comma and read more pairs.
+       ((eqv? ch #\,)
+        (read-char port)
+        (cond
+         (added (loop pairs #f))
+         (else (json-exception port))))
+       ;; Invalid object.
+       (else (json-exception port))))))
+
+;;
+;; Array parsing helpers
+;;
+
+(define (json-read-array port null ordered)
+  (expect-delimiter port #\[)
+  (skip-whitespaces port)
+  (cond
+   ;; Special case when array is empty.
+   ((eqv? (peek-char port) #\])
+    (read-char port)
+    #())
+   (else
+    ;; Read first element in array.
+    (let loop ((values (list (json-read port null ordered))))
+      (skip-whitespaces port)
+      (let ((ch (peek-char port)))
+        (cond
+         ;; Unexpected EOF.
+         ((eof-object? ch) (json-exception port))
+         ;; Handle comma (if there's a comma there should be another element).
+         ((eqv? ch #\,)
+          (read-char port)
+          (loop (cons (json-read port null ordered) values)))
+         ;; End of array.
+         ((eqv? ch #\])
+          (read-char port)
+          (list->vector (reverse! values)))
+         ;; Anything else other than comma and end of array is wrong.
+         (else (json-exception port))))))))
+
+;;
+;; String parsing helpers
+;;
+
+(define (read-hex-digit->integer port)
+  (let ((ch (read-char port)))
+    (cond
+     ((eqv? ch #\0) 0)
+     ((eqv? ch #\1) 1)
+     ((eqv? ch #\2) 2)
+     ((eqv? ch #\3) 3)
+     ((eqv? ch #\4) 4)
+     ((eqv? ch #\5) 5)
+     ((eqv? ch #\6) 6)
+     ((eqv? ch #\7) 7)
+     ((eqv? ch #\8) 8)
+     ((eqv? ch #\9) 9)
+     ((or (eqv? ch #\A) (eqv? ch #\a)) 10)
+     ((or (eqv? ch #\B) (eqv? ch #\b)) 11)
+     ((or (eqv? ch #\C) (eqv? ch #\c)) 12)
+     ((or (eqv? ch #\D) (eqv? ch #\d)) 13)
+     ((or (eqv? ch #\E) (eqv? ch #\e)) 14)
+     ((or (eqv? ch #\F) (eqv? ch #\f)) 15)
+     (else (json-exception port)))))
+
+(define (read-unicode-value port)
+  (+ (* 4096 (read-hex-digit->integer port))
+      (* 256 (read-hex-digit->integer port))
+      (* 16 (read-hex-digit->integer port))
+      (read-hex-digit->integer port)))
+
+;; Unicode codepoint with surrogates is:
+;;   10000 + (high - D800) + (low - DC00)
+;; which is equivalent to:
+;;  (high << 10) + low - 35FDC00
+;; see
+;;   https://github.com/aconchillo/guile-json/issues/58#issuecomment-662744070
+(define (json-surrogate-pair->unicode high low)
+  (+ (* high #x400) low #x-35FDC00))
+
+(define (read-unicode-char port)
+  (let ((codepoint (read-unicode-value port)))
+    (cond
+     ;; Surrogate pairs. `codepoint` already contains the higher surrogate
+     ;; (between D800 and DC00) . At this point we are expecting another
+     ;; \uXXXX that holds the lower surrogate (between DC00 and DFFF).
+     ((and (>= codepoint #xD800) (< codepoint #xDC00))
+      (expect-string port "\\u" #f)
+      (let ((low-surrogate (read-unicode-value port)))
+        (if (and (>= low-surrogate #xDC00) (< low-surrogate #xE000))
+            (integer->char (json-surrogate-pair->unicode codepoint low-surrogate))
+            (json-exception port))))
+     ;; Reserved for surrogates (we just need to check starting from the low
+     ;; surrogates).
+     ((and (>= codepoint #xDC00) (< codepoint #xE000))
+      (json-exception port))
+     (else (integer->char codepoint)))))
+
+(define (read-control-char port)
+  (let ((ch (read-char port)))
+    (cond
+     ((eqv? ch #\") #\")
+     ((eqv? ch #\\) #\\)
+     ((eqv? ch #\/) #\/)
+     ((eqv? ch #\b) #\bs)
+     ((eqv? ch #\f) #\ff)
+     ((eqv? ch #\n) #\lf)
+     ((eqv? ch #\r) #\cr)
+     ((eqv? ch #\t) #\ht)
+     ((eqv? ch #\u) (read-unicode-char port))
+     (else (json-exception port)))))
+
+(define (json-read-string port)
+  (expect-delimiter port #\")
+  (let loop ((chars '()) (ch (read-char port)))
+    (cond
+     ;; Unexpected EOF.
+     ((eof-object? ch) (json-exception port))
+     ;; Unescaped control characters are not allowed.
+     ((control-char? ch) (json-exception port))
+     ;; End of string.
+     ((eqv? ch #\") (reverse-list->string chars))
+     ;; Escaped characters.
+     ((eqv? ch #\\)
+      (loop (cons (read-control-char port) chars) (read-char port)))
+     ;; All other characters.
+     (else
+      (loop (cons ch chars) (read-char port))))))
+
+;;
+;; Booleans and null parsing helpers
+;;
+
+(define (json-read-true port)
+  (expect-string port "true" #t))
+
+(define (json-read-false port)
+  (expect-string port "false" #f))
+
+(define (json-read-null port null)
+  (expect-string port "null" null))
+
+;;
+;; Main parser functions
+;;
+
+(define (json-read port null ordered)
+  (skip-whitespaces port)
+  (let ((ch (peek-char port)))
+    (cond
+     ;; Unexpected EOF.
+     ((eof-object? ch) (json-exception port))
+     ;; Read JSON values.
+     ((eqv? ch #\t) (json-read-true port))
+     ((eqv? ch #\f) (json-read-false port))
+     ((eqv? ch #\n) (json-read-null port null))
+     ((eqv? ch #\{) (json-read-object port null ordered))
+     ((eqv? ch #\[) (json-read-array port null ordered))
+     ((eqv? ch #\") (json-read-string port))
+     ;; Anything else should be a number.
+     (else (json-read-number port)))))
+
+;;
+;; Public procedures
+;;
+
+(define* (json->scm #:optional (port (current-input-port))
+                    #:key (null 'null) (ordered #f) (concatenated #f))
+  "Parse a JSON document into native. Takes one optional argument,
+@var{port}, which defaults to the current input port from where the JSON
+document is read. It also takes a few of keyword arguments: @{null}: value for
+JSON's null, it defaults to the 'null symbol, @{ordered} to indicate whether
+JSON objects order should be preserved or not (the default) and @{concatenated}
+which can be used to tell the parser that more JSON documents might come after a
+properly parsed document."
+  (let loop ((value (json-read port null ordered)))
+    ;; Skip any trailing whitespaces.
+    (skip-whitespaces port)
+    (cond
+     ;; If we reach the end the parsing succeeded.
+     ((eof-object? (peek-char port)) value)
+     ;; If there's anything else other than the end, check if user wants to keep
+     ;; parsing concatenated valid JSON documents, otherwise parser fails.
+     (else
+      (cond (concatenated value)
+            (else (json-exception port)))))))
+
+(define* (json-string->scm str #:key (null 'null) (ordered #f))
+  "Parse a JSON document into native. Takes a string argument,
+@var{str}, that contains the JSON document. It also takes a couple of keyword
+argument: @{null}: value for JSON's null, it defaults to the 'null symbol and
+@{ordered} to indicate whether JSON objects order should be preserved or
+not (the default)."
+  (call-with-input-string str (lambda (p) (json->scm p #:null null #:ordered ordered))))
+
+(define* (json-seq->scm #:optional (port (current-input-port))
+                        #:key (null 'null) (ordered #f)
+                        (handle-truncate 'skip) (truncated-object 'truncated))
+  "Lazy parse a JSON text sequence from the port @var{port}.
+This procedure returns a stream of parsed documents. The optional argument
+@var{port} defines the port to read from and defaults to the current input
+port. It also takes a few keyword arguments: @{null}: value for JSON's null
+(defaults to the 'null symbol), @{ordered} to indicate whether JSON objects
+order should be preserved or not (the default), @{handle-truncate}: defines how
+to handle data loss, @{truncated-object}: used to replace unparsable
+objects. Allowed values for @{handle-truncate} argument are 'throw (throw an
+exception), 'stop (stop parsing and end the stream), 'skip (default, skip
+corrupted fragment and return the next entry), 'replace (skip corrupted fragment
+and return @{truncated-object} instead)."
+  (letrec ((handle-truncation
+            (case handle-truncate
+              ((throw) json-exception)
+              ((stop) (const (eof-object)))
+              ((skip)
+               (lambda (port)
+                 (read-delimited "\x1e" port 'peek)
+                 (read-entry port)))
+              ((replace)
+               (lambda (port)
+                 (read-delimited "\x1e" port 'peek)
+                 truncated-object))))
+           (read-entry
+            (lambda (port)
+              (let ((ch (read-char port)))
+                (cond
+                 ((eof-object? ch) ch)
+                 ((not (eqv? ch #\rs))
+                  (handle-truncation port))
+                 (else
+                  (skip-record-separators port)
+                  (catch 'json-invalid
+                    (lambda ()
+                      (let ((next (json-read port null ordered)))
+                        (if (eqv? #\lf (peek-char port))
+                            (begin
+                              (read-char port)
+                              next)
+                            (handle-truncation port))))
+                    (lambda (_ port)
+                      (handle-truncation port)))))))))
+    (port->stream port read-entry)))
+
+(define* (json-seq-string->scm str #:key (null 'null) (ordered #f)
+                               (handle-truncate 'skip) (truncated-object 'truncated))
+  "Lazy parse a JSON text sequence from the string @var{str}.
+This procedure returns a stream of parsed documents and also takes the same
+keyword arguments as @code{json-seq->scm}."
+  (call-with-input-string str
+    (lambda (p)
+      (json-seq->scm p #:null null #:ordered ordered
+                     #:handle-truncate handle-truncate
+                     #:truncated-object truncated-object))))
+
+;;; (json parser) ends here