exprs : expr
          | exprs COMMA expr

def p_exprs(p):
    """
    exprs : expr
          | exprs COMMA expr
    """
    if len(p) == 2:
        p[0] = node.expr_list([p[1]])
    elif len(p) == 4:
        p[0] = p[1]
        p[0].append(p[3])
    else:
        assert (0)
    assert isinstance(p[0], node.expr_list)