/// <summary>
 /// Annotate the specified <see cref="ElementDefinition"/> instance in the <see cref="StructureDefinition.Differential"/> component
 /// with a reference to the associated <see cref="ElementDefinition"/> instance in the <see cref="StructureDefinition.Snapshot"/> component.
 /// </summary>
 /// <param name="diffElemDef"></param>
 /// <param name="snapElemDef"></param>
 internal static void SetSnapshotElementAnnotation(this ElementDefinition diffElemDef, ElementDefinition snapElemDef)
 {
     diffElemDef?.AddAnnotation(new SnapshotElementDefinitionAnnotation(snapElemDef));
 }