matrix : LBRACKET RBRACKET
              | LBRACKET concat_list RBRACKET
              | LBRACKET concat_list SEMI RBRACKET
              | LBRACKET expr_list RBRACKET
              | LBRACKET expr_list SEMI RBRACKET

def p_matrix(p):
    """matrix : LBRACKET RBRACKET
              | LBRACKET concat_list RBRACKET
              | LBRACKET concat_list SEMI RBRACKET
              | LBRACKET expr_list RBRACKET
              | LBRACKET expr_list SEMI RBRACKET
    """
    if len(p) == 3:
        p[0] = node.matrix()
    else:
        p[0] = node.matrix(p[2])