get_parent
Description:
Retrieves the parent node of the given this.
Parameters:
| this | 
           the node to query  | 
      
Returns:
| 
           the parent node, or `NULL` if this is the root node  | 
      
Retrieves the parent node of the given this.
| this | 
           the node to query  | 
      
| 
           the parent node, or `NULL` if this is the root node  |