diff --git a/reactor/Core/src/jetbrains/mps/logic/reactor/core/RuleIndex.kt b/reactor/Core/src/jetbrains/mps/logic/reactor/core/RuleIndex.kt
index 932ed19f..743991b5 100644
--- a/reactor/Core/src/jetbrains/mps/logic/reactor/core/RuleIndex.kt
+++ b/reactor/Core/src/jetbrains/mps/logic/reactor/core/RuleIndex.kt
@@ -56,6 +56,7 @@ class RuleIndex(): Iterable<Rule>, RuleLookup
 
     private val symbol2index = HashMap<ConstraintSymbol, ArgumentRuleIndex>()
 
+    // TODO: is linked hashmap really necessary? might be inefficient
     private val tag2rule = LinkedHashMap<Any, Rule>()
 
     private val tag2bit = TObjectIntHashMap<Any>()