Sets whether the element is considered the "main" element for the 
SpotPanel, around which all of the sibling elements are positioned.
 
            
            
            Syntax
            
        
            Parameters
- d
 
- this should be a child of a SpotPanel
 - v
 
- only one such child of the SpotPanel should have this property set to true
 
            
             
            
            
            
            
            
            
            
            
            
            
            
            See Also