don't edit openxmlcompositelement

Author: mikeebowen

src/DocumentFormat.OpenXml.Framework/OpenXmlCompositeElement.cs

@@ -3,6 +3,7 @@
 
 using DocumentFormat.OpenXml.Framework;
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;