diff --git a/sys/thing/properties.h b/sys/thing/properties.h
index 2ccf20e..c472af5 100644
--- a/sys/thing/properties.h
+++ b/sys/thing/properties.h
@@ -49,6 +49,10 @@
 // filtered property access
 public mixed SetProp(string name, mixed Value, int drop_priv = extern_call());
 public mixed QueryProp(string name);
+public mixed AddToProp(string pname,
+                       <int|float|string|mixed*|mapping|bytes> add_value);
+public mixed SubFromProp(string pname,
+                       <int|float|string|mixed*|mapping|bytes> sub_value);
 
 // global property handling
 public void SetProperties(mapping props);