(p)
opt_parameter_list : parameter_list
                        |   parameter_list commas
                        |   empty
opt_parameter_list : parameter_list
                        |   parameter_list commas
                        |   empty
opt_parameter_list : parameter_list | parameter_list commas | empty
def p_opt_parameter_list(p):
    '''opt_parameter_list : parameter_list
                        |   parameter_list commas
                        |   empty
       '''
    if p[1] != None:
        p[0] = p[1]
    else:
       p[0] = []
def p_opt_parameter_list ( p ) : if p [ 1 ] != None : p [ 0 ] = p [ 1 ] else : p [ 0 ] = [ ]