diff --git a/src/main/scala/leon/synthesis/strategies/CostBasedStrategy.scala b/src/main/scala/leon/synthesis/strategies/CostBasedStrategy.scala
index 47bf215b6c2c2975f11f59a63a807b640bd03408..70ccc90ab0175828a65866f7a5c347c0b2e18dd5 100644
--- a/src/main/scala/leon/synthesis/strategies/CostBasedStrategy.scala
+++ b/src/main/scala/leon/synthesis/strategies/CostBasedStrategy.scala
@@ -7,8 +7,8 @@ package strategies
 import synthesis.graph._
 
 class CostBasedStrategy(ctx: LeonContext, cm: CostModel) extends Strategy {
-  var bestSols = Map[Node, Option[Solution]]()
-  var bestCosts = Map[Node, Cost]()
+  private var bestSols = Map[Node, Option[Solution]]()
+  private var bestCosts = Map[Node, Cost]()
 
   override def init(root: RootNode): Unit = {
     super.init(root)
diff --git a/src/main/scala/leon/utils/GrowableIterable.scala b/src/main/scala/leon/utils/GrowableIterable.scala
index b20054a32b033039f72629858b9839fa0e99175a..072ca71197fcacf752df8b6546243fa579a24f3c 100644
--- a/src/main/scala/leon/utils/GrowableIterable.scala
+++ b/src/main/scala/leon/utils/GrowableIterable.scala
@@ -5,10 +5,7 @@ import scala.collection.mutable.ArrayBuffer
 class GrowableIterable[T](init: Seq[T], growth: Iterator[T], canGrow: () => Boolean) extends Iterable[T] {
   private val buffer = new ArrayBuffer[T]() ++ init
 
-  def +=(more: T) = buffer += more
-  def ++=(more: Seq[T]) = buffer ++= more
-
-  val cachingIterator = new Iterator[T] {
+  private val cachingIterator = new Iterator[T] {
     def hasNext = canGrow() && growth.hasNext
 
     def next() = {
@@ -18,6 +15,9 @@ class GrowableIterable[T](init: Seq[T], growth: Iterator[T], canGrow: () => Bool
     }
   }
 
+  def +=(more: T) = buffer += more
+  def ++=(more: Seq[T]) = buffer ++= more
+
   def iterator: Iterator[T] = {
     buffer.iterator ++ cachingIterator
   }