diff --git a/core/src/main/kotlin/tw/xcc/gumtree/api/tree/PriorityTreeList.kt b/core/src/main/kotlin/tw/xcc/gumtree/api/tree/PriorityTreeList.kt
index 8bab81d..753434e 100644
--- a/core/src/main/kotlin/tw/xcc/gumtree/api/tree/PriorityTreeList.kt
+++ b/core/src/main/kotlin/tw/xcc/gumtree/api/tree/PriorityTreeList.kt
@@ -1,8 +1,6 @@
 package tw.xcc.gumtree.api.tree
 
-import tw.xcc.gumtree.model.BasicTree
-
-internal interface PriorityTreeList<T : BasicTree<T>> {
+internal interface PriorityTreeList<in T : Tree> {
     /**
      * Inserts the node n in the list.
      * */
@@ -14,15 +12,15 @@ internal interface PriorityTreeList<T : BasicTree<T>> {
     fun open(tree: T)
 
     /**
-     * Returns and removes the set of all nodes which has a priority equals to [peekMax]
+     * Returns and removes the set of all nodes that has a priority equals to [peekMax]
      * */
-    fun pop(): List<T>
+    fun pop(): List<Tree>
 
     /**
      * Returns and removes the set of all nodes which has a priority equals to [peekMax],
      * and inserts all the children of these nodes in the list.
      * */
-    fun popOpen(): List<T>
+    fun popOpen(): List<Tree>
 
     /**
      * Returns the highest priority value of the list.
diff --git a/core/src/main/kotlin/tw/xcc/gumtree/api/tree/TreeMappingStorage.kt b/core/src/main/kotlin/tw/xcc/gumtree/api/tree/TreeMappingStorage.kt
new file mode 100644
index 0000000..196ceaf
--- /dev/null
+++ b/core/src/main/kotlin/tw/xcc/gumtree/api/tree/TreeMappingStorage.kt
@@ -0,0 +1,27 @@
+package tw.xcc.gumtree.api.tree
+
+internal interface TreeMappingStorage<in T : Tree> {
+    fun addMappingOf(mapping: Pair<T, T>)
+
+    fun addMappingRecursivelyOf(mapping: Pair<T, T>)
+
+    fun removeMappingOf(mapping: Pair<T, T>)
+
+    fun getMappingOfLeft(left: T): Tree?
+
+    fun getMappingOfRight(right: T): Tree?
+
+    fun isLeftMapped(left: T): Boolean
+
+    fun isAnyOfLeftsUnMapped(lefts: Iterable<T>): Boolean
+
+    fun isRightMapped(right: T): Boolean
+
+    fun isAnyOfRightsUnMapped(rights: Iterable<T>): Boolean
+
+    fun areBothUnMapped(mapping: Pair<T, T>): Boolean
+
+    fun hasUnMappedDescendentOfLeft(left: T): Boolean
+
+    fun hasUnMappedDescendentOfRight(right: T): Boolean
+}
\ No newline at end of file
diff --git a/core/src/main/kotlin/tw/xcc/gumtree/model/MappingStorage.kt b/core/src/main/kotlin/tw/xcc/gumtree/model/MappingStorage.kt
index 15f79c8..6022ba7 100644
--- a/core/src/main/kotlin/tw/xcc/gumtree/model/MappingStorage.kt
+++ b/core/src/main/kotlin/tw/xcc/gumtree/model/MappingStorage.kt
@@ -4,11 +4,12 @@ import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.runBlocking
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
+import tw.xcc.gumtree.api.tree.TreeMappingStorage
 
 /**
  * The storage for saving the references of the mapping between two GumTrees. (Left and Right)
  * */
-class MappingStorage {
+class MappingStorage : TreeMappingStorage<GumTree> {
     private val mappingLR = mutableMapOf<GumTree, GumTree>()
     private val mappingRL = mutableMapOf<GumTree, GumTree>()
 
@@ -97,62 +98,62 @@ class MappingStorage {
             !mappingLR.containsKey(mapping.first) && !mappingRL.containsKey(mapping.second)
         }
 
-    fun addMappingOf(mapping: Pair<GumTree, GumTree>) =
+    override fun addMappingOf(mapping: Pair<GumTree, GumTree>) =
         runBlocking(Dispatchers.Default) {
             addMappingImpl(mapping)
         }
 
-    fun addMappingRecursivelyOf(mapping: Pair<GumTree, GumTree>) =
+    override fun addMappingRecursivelyOf(mapping: Pair<GumTree, GumTree>) =
         runBlocking(Dispatchers.Default) {
             addMappingRecursivelyImpl(mapping)
         }
 
-    fun removeMappingOf(mapping: Pair<GumTree, GumTree>) =
+    override fun removeMappingOf(mapping: Pair<GumTree, GumTree>) =
         runBlocking(Dispatchers.Default) {
             removeMappingImpl(mapping)
         }
 
-    fun getMappingOfLeft(left: GumTree): GumTree? =
+    override fun getMappingOfLeft(left: GumTree): GumTree? =
         runBlocking(Dispatchers.Default) {
             extractMappedTreeOf(left, mappingLR)
         }
 
-    fun getMappingOfRight(right: GumTree): GumTree? =
+    override fun getMappingOfRight(right: GumTree): GumTree? =
         runBlocking(Dispatchers.Default) {
             extractMappedTreeOf(right, mappingRL)
         }
 
-    fun isLeftMapped(left: GumTree): Boolean =
+    override fun isLeftMapped(left: GumTree): Boolean =
         runBlocking(Dispatchers.Default) {
             isMappingExistsIn(mappingLR, left)
         }
 
-    fun isAnyOfLeftsUnMapped(lefts: Iterable<GumTree>): Boolean =
+    override fun isAnyOfLeftsUnMapped(lefts: Iterable<GumTree>): Boolean =
         runBlocking(Dispatchers.Default) {
             isAnyTreeNotExistsIn(mappingLR, lefts)
         }
 
-    fun isRightMapped(right: GumTree): Boolean =
+    override fun isRightMapped(right: GumTree): Boolean =
         runBlocking(Dispatchers.Default) {
             isMappingExistsIn(mappingRL, right)
         }
 
-    fun isAnyOfRightsUnMapped(rights: Iterable<GumTree>): Boolean =
+    override fun isAnyOfRightsUnMapped(rights: Iterable<GumTree>): Boolean =
         runBlocking(Dispatchers.Default) {
             isAnyTreeNotExistsIn(mappingRL, rights)
         }
 
-    fun areBothUnMapped(mapping: Pair<GumTree, GumTree>): Boolean =
+    override fun areBothUnMapped(mapping: Pair<GumTree, GumTree>): Boolean =
         runBlocking(Dispatchers.Default) {
             areBothUnMappedImpl(mapping)
         }
 
-    fun hasUnMappedDescendentOfLeft(left: GumTree): Boolean =
+    override fun hasUnMappedDescendentOfLeft(left: GumTree): Boolean =
         runBlocking(Dispatchers.Default) {
             hasUnMappedDescendent(left, mappingLR)
         }
 
-    fun hasUnMappedDescendentOfRight(right: GumTree): Boolean =
+    override fun hasUnMappedDescendentOfRight(right: GumTree): Boolean =
         runBlocking(Dispatchers.Default) {
             hasUnMappedDescendent(right, mappingRL)
         }