Retrieves a filter to restrict elements to be checked. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2012 
Syntax
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Parameters
- document
-  Type:  Autodesk.Revit.DB Document  
 Document for which performance problems are being checked.
Return Value
The filter to restrict elements to be checked.Remarks
 If the rule needs to be executed on individual elements, it should return a filter that defines what elements the rule should apply to. Otherwise, nothing (i.e. null) should be returned. If filter is returned, it should stay valid till the end of the check.