diff --git a/doc/output-format.md b/doc/output-format.md
index 3d91f28..15300cf 100644
--- a/doc/output-format.md
+++ b/doc/output-format.md
@@ -190,12 +190,14 @@ axiomRule(axiomRule(<number>)).
 precondition(<axiom rule>, <variable>, <value>).
 
 % defines that after applying <axiom rule>, <variable> is assigned <value>
-postcondition(<axiom rule>, <variable>, <value>).
+postcondition(<axiom rule>, effect(unconditional), <variable>, <value>).
 ```
 
 Axiom rules are similar to [actions](#actions) in that they modify [variables](#variables) if certain preconditions are satisfied.
 However, axiom rules must be applied *immediately* as soon as their preconditions are satisfied.
 
+The second argument of `postcondition`, `effect(unconditional)`, is not used and exists only for consistency with [actions](#actions).
+
 Axiom rule facts are only present with SAS input programs and not PDDL.
 
 Axiom rules contain essential information in order to find plans correctly.