From 582bb6f37402960bb5d52aff9c7651ff6e8b1e29 Mon Sep 17 00:00:00 2001
From: David Kastrup <dak@gnu.org>
Date: Sun, 9 Dec 2012 13:19:38 +0100
Subject: [PATCH] Issue 2981: Let \set ... = *undefined* unset values rather
 than overwriting.

Also makes \once\set deal properly with a value that is not previously
defined in the current context.
---
 lily/context.cc           |  6 ++++++
 lily/lily-guile.cc        |  2 +-
 lily/property-iterator.cc | 13 ++++++++++---
 3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/lily/context.cc b/lily/context.cc
index 4d949e8379..90b41c155e 100644
--- a/lily/context.cc
+++ b/lily/context.cc
@@ -228,6 +228,12 @@ Context::set_property_from_event (SCM sev)
   if (scm_is_symbol (sym))
     {
       SCM val = ev->get_property ("value");
+
+      if (SCM_UNBNDP (val)) {
+        unset_property (sym);
+        return;
+      }
+          
       bool ok = true;
       if (val != SCM_EOL)
         ok = type_check_assignment (sym, val, ly_symbol2scm ("translation-type?"));
diff --git a/lily/lily-guile.cc b/lily/lily-guile.cc
index e8380324fb..3fd0e890a0 100644
--- a/lily/lily-guile.cc
+++ b/lily/lily-guile.cc
@@ -62,7 +62,7 @@ ly_scm_write_string (SCM s)
 SCM
 ly_quote_scm (SCM s)
 {
-  return scm_list_n (ly_symbol2scm ("quote"), s, SCM_UNDEFINED);
+  return scm_list_2 (ly_symbol2scm ("quote"), s);
 }
 
 string
diff --git a/lily/property-iterator.cc b/lily/property-iterator.cc
index af1b38d159..6352f16399 100644
--- a/lily/property-iterator.cc
+++ b/lily/property-iterator.cc
@@ -35,14 +35,21 @@ Property_iterator::process (Moment mom)
 {
   Context *o = get_outlet ();
   Music *m = get_music ();
-  SCM previous_value = o->get_property (m->get_property ("symbol"));
+  bool once = to_boolean (m->get_property ("once"));
+  SCM symbol = m->get_property ("symbol");
+  SCM previous_value = SCM_UNDEFINED;
+  if (once) {
+    Context *w = o->where_defined (symbol, &previous_value);
+    if (o != w)
+      previous_value = SCM_UNDEFINED;
+  }
   send_stream_event (o, "SetProperty", m->origin (),
-                     ly_symbol2scm ("symbol"), m->get_property ("symbol"),
+                     ly_symbol2scm ("symbol"), symbol,
                      ly_symbol2scm ("value"), m->get_property ("value"));
 
   /* For \once \set install a finalization hook to reset the property to the
    * previous value after the timestep */
-  if (to_boolean (m->get_property ("once")))
+  if (once)
     {
       Global_context *tg = get_outlet ()->get_global_context ();
       tg->add_finalization (scm_list_n (once_finalization_proc,
-- 
2.39.2