Returns the id  of the target that is set as "head"
        @rtype: string
        @return: the target id (or None) of the head target

def get_id_head(self):
        '''
        Returns the id  of the target that is set as "head"
        @rtype: string
        @return: the target id (or None) of the head target
        '''
        id_head = None
        for target_node in self:
            if target_node.is_head():
                id_head = target_node.get_id()
                break
        return id_head