1 files changed, 1 insertions, 1 deletions

diff --git a/subprojects/frontend/src/language/problem.grammar b/subprojects/frontend/src/language/problem.grammar
index 0d3f0e3a..ffae220c 100644
--- a/subprojects/frontend/src/language/problem.grammar
+++ b/subprojects/frontend/src/language/problem.grammar
@@ -66,7 +66,7 @@ statement {
   RuleDefinition {
     (ckw<"decision"> | ckw<"propagation">)? kw<"rule">
     RuleName ParameterList<Parameter>?
-    RuleBody { ":" sep<OrOp, Conjunction> "==>" sep<OrOp, Consequent> "." }
+    RuleBody { ("<->" sep<OrOp, Conjunction>)? ("==>" sep<OrOp, Consequent>)? "." }
   } |
   AtomDeclaration {
     kw<"declare">? ckw<"atom"> sep<",", AtomNodeName> "."