Make SkylarkClassObject "Bazel-specific".
This in preparation to DeclaredProviders implementation.
--
MOS_MIGRATED_REVID=129420617
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Concatable.java b/src/main/java/com/google/devtools/build/lib/syntax/Concatable.java
new file mode 100644
index 0000000..81f489f
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Concatable.java
@@ -0,0 +1,36 @@
+// Copyright 2016 The Bazel Authors. All Rights Reserved.
+//
+// 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.devtools.build.lib.syntax;
+
+import com.google.devtools.build.lib.events.Location;
+import javax.annotation.Nullable;
+
+/**
+ * Skylark values that support '+' operator should implement this interface.
+ */
+public interface Concatable {
+
+ /**
+ * Implements 'plus' operator on ClassObjects.
+ */
+ interface Concatter {
+ Concatable concat(Concatable lval, Concatable rval, Location loc) throws EvalException;
+ }
+
+ /* Returns a concatter for this {@link Concatable}.
+ * Two {@link Concatable}s can be added together if their concatters are equal.
+ */
+ @Nullable
+ Concatter getConcatter();
+}
