get_source
  
  Description:
  public unowned SetSource get_source (
Holder holder)
  
 
  
    Finds a SetSource which contains the 
      DataModel restricting the possible values of holder, don't modify
      the returned structure.
    Parameters:
    
      
        | this | a Set object | 
      
        | holder | a Holder object | 
    
    Returns:
    
      
        | the requested SetSource or null |