action_precond_list : action_precond_list action_precond_def
                               | action_precond_def

def p_action_precond_list(self, p):
        '''action_precond_list : action_precond_list action_precond_def
                               | action_precond_def'''
        if len(p) == 3:
            p[1].append(p[2])
            p[0] = p[1]
        elif len(p) == 2:
            p[0] = [p[1]]