[S2GRAPH-229] 'Step' abstraction for combinable queries

JIRA:
  [S2GRAPH-229] https://issues.apache.org/jira/browse/S2GRAPH-229

Pull Request:
  Closes #178

Author
  daewon <daewon@apache.org>
diff --git a/CHANGES b/CHANGES
index fead999..da32f18 100644
--- a/CHANGES
+++ b/CHANGES
@@ -104,6 +104,7 @@
     * [S2GRAPH-162] - Update year in the NOTICE file.
     * [S2GRAPH-211] - Include 's2jobs' test in CI
     * [S2GRAPH-212] - Fix broken markdown on README.md.
+    * [S2GRAPH-229] - 'Step' abstraction for combinable queries
 
 Release 0.1.0 - Released
 
diff --git a/s2core/src/main/scala/org/apache/s2graph/core/step/IStep.scala b/s2core/src/main/scala/org/apache/s2graph/core/step/IStep.scala
index e0e23f6..e700bf8 100644
--- a/s2core/src/main/scala/org/apache/s2graph/core/step/IStep.scala
+++ b/s2core/src/main/scala/org/apache/s2graph/core/step/IStep.scala
@@ -1,3 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.s2graph.core.step
 
 import org.apache.s2graph.core._
diff --git a/s2core/src/test/scala/org/apache/s2graph/core/step/GraphStepTest.scala b/s2core/src/test/scala/org/apache/s2graph/core/step/GraphStepTest.scala
index 96e49a0..0096212 100644
--- a/s2core/src/test/scala/org/apache/s2graph/core/step/GraphStepTest.scala
+++ b/s2core/src/test/scala/org/apache/s2graph/core/step/GraphStepTest.scala
@@ -1,3 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.s2graph.core.step
 
 import org.apache.s2graph.core.Integrate.IntegrateCommon
diff --git a/s2core/src/test/scala/org/apache/s2graph/core/step/StepTest.scala b/s2core/src/test/scala/org/apache/s2graph/core/step/StepTest.scala
index f29a346..3acc9d6 100644
--- a/s2core/src/test/scala/org/apache/s2graph/core/step/StepTest.scala
+++ b/s2core/src/test/scala/org/apache/s2graph/core/step/StepTest.scala
@@ -1,3 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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 org.apache.s2graph.core.step
 
 import org.scalatest.{BeforeAndAfterEach, FunSuite, Matchers}