One day I will learn to include all required classes in the commit.

src/main/java/net/minecraftforge/client/model/IModelSimpleProperties.java

@@ -0,0 +1,10 @@
+package net.minecraftforge.client.model;
+
+/**
+ * Implement this if the model can process "smooth_lighting" or "gui3d" attributes from the json.
+ */
+public interface IModelSimpleProperties<M extends IModelSimpleProperties<M>> extends IModel
+{
+    M smoothLighting(boolean value);
+    M gui3d(boolean value);
+}