Adds a list of roles to the predicate
        @type list_of_roles: list
        @param list_of_roles: list of roles

def add_roles(self, list_of_roles):
        """
        Adds a list of roles to the predicate
        @type list_of_roles: list
        @param list_of_roles: list of roles
        """
        for role in list_of_roles:
            role_node = role.get_node()
            self.node.append(role_node)