+static Protected_scm modification_callback (SCM_EOL);
+static Protected_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.")