refactor include system

private_includes/base/include_from_song.ily

@@ -0,0 +1,15 @@
+includeFromSong =
+#(define-void-function (filename) (string?)
+  (let ((noDefaultOutputBackup noDefaultOutput))
+    (set! noDefaultOutput #t)
+    (ly:parser-parse-string (ly:parser-clone)
+      (ly:format "\\include \"~a\""
+      (string-append
+        (dirname (dirname (dirname (dirname (current-filename)))))
+        file-name-separator-string
+        "lilypond-song-includes"
+        file-name-separator-string
+        "liedbausteine"
+        file-name-separator-string
+        filename)))
+    (set! noDefaultOutput noDefaultOutputBackup)))