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