diff --git a/subprojects/frontend/src/language/problem.grammar b/subprojects/frontend/src/language/problem.grammar
index 1af17c36..254029cd 100644
--- a/subprojects/frontend/src/language/problem.grammar
+++ b/subprojects/frontend/src/language/problem.grammar
@@ -54,7 +54,7 @@ statement {
   PredicateDefinition {
     ((kw<"error"> | kw<"partial"> | ckw<"shadow">)* kw<"pred"> | kw<"error">)
     RelationName ParameterList<Parameter>?
-    (kw<"subsets"> sep1<",", RelationName>)*
+    (ckw<"subsets"> sep1<",", RelationName>)*
     PredicateBody { ("<->" sep<OrOp, Conjunction>)? "." }
   } |
   // FunctionDefinition {
@@ -91,8 +91,10 @@ FeatureDeclaration {
   ("[" Multiplicity? "]")?
   RelationName
   (
-    kw<"opposite"> RelationName |
-    kw<"subsets"> sep1<",", RelationName>
+    !feature RelationOptions[@dynamicPrecedence=1] {
+      ckw<"opposite"> RelationName |
+      ckw<"subsets"> sep1<",", RelationName>
+    }
   )*
   ";"?
 }
diff --git a/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext b/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
index 4e266b18..283afae2 100644
--- a/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
+++ b/subprojects/language/src/main/java/tools/refinery/language/Problem.xtext
@@ -301,7 +301,7 @@ NonContainmentQualifiedName hidden():
 	(NonContainmentIdentifier | "::" Identifier) (QUALIFIED_NAME_SEPARATOR Identifier)*;
 
 Identifier:
-	NonContainmentIdentifier | "contains" | "container";
+	NonContainmentIdentifier | "contains" | "container" | "opposite" | "subsets";
 
 NonContainmentIdentifier:
 	ID | QUOTED_ID | "atom" | "multi" | "problem" | "module" |