switch (tok)
        {
        case ID:
              /* <statement> ::= ID := <expression> ; */
              target = ident();
            match(ASSIGNOP);
              expression();
              assign(target, source);
              match(SEMICOLON);
                break;

        case READ:
              /* <statement> ::= READ ( <id list> ) ; */
              match(READ);
            match(LPAREN);
              id_list();
              match(RPAREN);
            match(SEMICOLON);
                break;

        case WRITE:
              /* <statement> ::= WRITE ( <expr list> ) ; */
              match(WRITE);
             match(LPAREN);
              expr_list();
              match(RPAREN);
             match(SEMICOLON);
                break;

        default:
                syntax_error(tok);
                break;
        }
}