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diff --git a/src/main/scala/com/google/gimd/query/Node.scala b/src/main/scala/com/google/gimd/query/Node.scala
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+// Copyright (C) 2009 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gimd.query
+
+import com.google.gimd.FieldSpecOne
+
+/**
+ * Base class for all nodes in Query AST.
+ *
+ * NOTE: As for now Node does only track the type (T) of the value that comes from
+ * evaluating expression held by Node given specific instance defined by UserType.
+ * However, Node does not track the type of UserType so it's possible that Node's
+ * expression would be applied to wrong instance. This will be fixed in a future.
+ */
+abstract class Node[T]
+
+/**
+ * Node that stores FieldSpec. This node acts as placeholder in query for a given field of specific instance
+ * defined by UserType.
+ */
+final case class FieldSpecOneNode[T](val fieldSpecOne: FieldSpecOne[_, T]) extends Node[T]
+
+/**
+ * Node that holds a constant value of type T.
+ */
+final case class ConstNode[T](val value: T) extends Node[T]
+
+/**
+ * Node that holds Predicate which (in turn) holds arbitrary function T => Boolean where T is a type
+ * of instance that UserType is bound to.
+ */
+final case class PredicateNode[T](p: Predicate[T]) extends Node[Boolean]