diff --git a/lib/lrama/grammar/parameterizing_rule_resolver.rb b/lib/lrama/grammar/parameterizing_rule_resolver.rb
index 5b8a8fda..9d92a412 100644
--- a/lib/lrama/grammar/parameterizing_rule_resolver.rb
+++ b/lib/lrama/grammar/parameterizing_rule_resolver.rb
@@ -10,15 +10,21 @@ def add_parameterizing_rule_builder(builder)
       end
 
       def defined?(name)
-        @parameterizing_rule_builders.any? { |builder| builder.name == name }
+        !rule_builders(name).empty?
       end
 
       def build_rules(token, rule_counter, lhs_tag, line)
-        builder = @parameterizing_rule_builders.select { |b| b.name == token.s_value }.last
+        builder = rule_builders(token.s_value).last
         raise "Unknown parameterizing rule #{token.s_value} at line #{token.line}" unless builder
 
         builder.build_rules(token, token.args, rule_counter, lhs_tag, line, @parameterizing_rule_builders)
       end
+
+      private
+
+      def rule_builders(name)
+        @parameterizing_rule_builders.select { |builder| builder.name == name }
+      end
     end
   end
 end
diff --git a/sig/lrama/grammar/parameterizing_rule_resolver.rbs b/sig/lrama/grammar/parameterizing_rule_resolver.rbs
index 1347f7a6..b24e06ab 100644
--- a/sig/lrama/grammar/parameterizing_rule_resolver.rbs
+++ b/sig/lrama/grammar/parameterizing_rule_resolver.rbs
@@ -7,6 +7,10 @@ module Lrama
       def add_parameterizing_rule_builder: (Grammar::ParameterizingRuleBuilder builder) -> void
       def defined?: (String) -> bool
       def build_rules: (Lexer::Token::InstantiateRule token, Counter rule_counter, untyped lhs_tag, Integer? line) -> Array[Grammar::ParameterizingRule]
+
+      private
+
+      def rule_builders: (String) -> Array[Grammar::ParameterizingRuleBuilder]
     end
   end
 end