Add a definition to be reused by instances. A definition is a single geometry object. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2015 
Syntax
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Parameters
- id
 -  Type:  System String  
ID of the definition to be added. Must be unique. 
- GNode
 -  Type:  Autodesk.Revit.DB GeometryObject  
Definition as a single Geometry object 
Exceptions
| Exception | Condition | 
|---|---|
| Autodesk.Revit.Exceptions ArgumentNullException | A non-optional argument was NULL |