adjust author sandbox

lieder/authors_sandbox/authors_sandbox.ly

@@ -11,6 +11,27 @@
   \header {
     authors = #'(
      ("kumpel" text melody))
+    year_text = "1600"
+    year_melody = "1600"
+  }
+  \markup { \print-songinfo }
+}
+
+\bookpart {
+  \header {
+    authors = #'(
+     ("kumpel" text melody))
+    year_text = "1600"
+    year_melody = "1800"
+  }
+  \markup { \print-songinfo }
+}
+
+\bookpart {
+  \header {
+    authors = #'(
+     ("kumpel" text melody))
+    year_melody = "1800"
   }
   \markup { \print-songinfo }
 }