diff --git a/database/rules.bolt b/database/rules.bolt
index 8dfe254..3b514f5 100644
--- a/database/rules.bolt
+++ b/database/rules.bolt
@@ -6,6 +6,10 @@ path /canvas/the-one-and-only/canvas {
   write() { true }
 }
 
+path /canvas/the-one-and-only/history/{id} {
+  write() { createOnly(this) }
+}
+
 type Canvas {
   depth: Integer;
   canvas: Square<Square<Square<Square<Square<Square<Square<Color>>>>>>>;
@@ -42,3 +46,9 @@ type ColorHistoryItem extends HistoryItem<Color> {}
 type CurrentTimestamp extends Number {
   validate() { this == now }
 }
+
+// Helper functions
+
+createOnly(value) {
+  prior(value) == null && value != null
+}
\ No newline at end of file