This property is used to retrieve all the mullion types in current system. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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Remarks
 This property returns a set of MullionType objects. These objects can be used change the type of a mullion from one type to another.