+#include "warn.hh"
+#include "protected-scm.hh"
+
+Protected_scm grob_property_callback_stack = SCM_EOL;
+
+extern bool debug_property_callbacks;
+
+#ifndef NDEBUG
+static void
+print_property_callback_stack ()
+{
+ int frame = 0;
+ for (SCM s = grob_property_callback_stack; scm_is_pair (s); s = scm_cdr (s))
+ message (_f ("%d: %s", frame++, ly_scm_write_string (scm_car (s)).c_str ()));
+}
+#endif
+
+static SCM modification_callback = SCM_EOL;
+static SCM cache_callback = SCM_EOL;
+
+
+/*
+
+FIXME: this should use ly:set-option interface instead.
+
+*/
+
+LY_DEFINE (ly_set_grob_modification_callback, "ly:set-grob-modification-callback",
+ 1, 0, 0, (SCM cb),
+ "Specify a procedure that will be called every time LilyPond"
+ " modifies a grob property. The callback will receive as"
+ " arguments the grob that is being modified, the name of the"
+ " C++ file in which the modification was requested, the line"
+ " number in the C++ file in which the modification was requested,"
+ " the name of the function in which the modification was"
+ " requested, the property to be changed, and the new value for"
+ " the property.")
+{
+ modification_callback = (ly_is_procedure (cb)) ? cb : SCM_BOOL_F;
+ return SCM_UNSPECIFIED;
+}
+
+LY_DEFINE (ly_set_property_cache_callback, "ly:set-property-cache-callback",
+ 1, 0, 0, (SCM cb),
+ "Specify a procedure that will be called whenever lilypond"
+ " calculates a callback function and caches the result. The"
+ " callback will receive as arguments the grob whose property it"
+ " is, the name of the property, the name of the callback that"
+ " calculated the property, and the new (cached) value of the"
+ " property.")
+{
+ cache_callback = (ly_is_procedure (cb)) ? cb : SCM_BOOL_F;
+ return SCM_UNSPECIFIED;
+}
+
+
+void
+Grob::instrumented_set_property (SCM sym, SCM v,
+ char const *file,
+ int line,
+ char const *fun)
+{
+#ifndef NDEBUG
+ if (ly_is_procedure (modification_callback))
+ scm_apply_0 (modification_callback,
+ scm_list_n (self_scm (),
+ scm_from_locale_string (file),
+ scm_from_int (line),
+ scm_from_locale_string (fun),
+ sym, v, SCM_UNDEFINED));
+#else
+ (void) file;
+ (void) line;
+ (void) fun;
+#endif
+
+ internal_set_property (sym, v);
+}