Present-to-absent check is necessary in assertion removal handler

syndicate/HOWITWORKS.md

@@ -444,8 +444,9 @@ check followed by zero or more (move, check) pairs.
         del_leaf (leafcache, _) v = leafcache -= v
         del_handler (bag, f_table) vs =
           bag[vs] -= 1
-          for f in f_table
-              f "-" vs
+          if vs not in bag then
+              for f in f_table
+                  f "-" vs
 
 **Definition.** The procedures `addAssertion` and `removeAssertion`
   install and remove an assertion `v` into the given index,