language While {
  nonterminal Stms, Stm, Exp, Base;

  Stms[one]  --> <Stm>        ;
      [more] --> <Stm> <Stms> ;
      
  Stm[ass]    --> <Lex.Var> := <Exp> ";"         ;
     [skip]   --> skip ";"                       ;
     [if]     --> if <Exp> then <Stm>            ;
     [ifelse] --> if <Exp> then <Stm> else <Stm> ;
     [while]  --> while <Exp> do <Stm>           ;
     [block]  --> { <Stms> }                     ;

  Exp[base] --> <Base>          ;
     [neg]  --> ! <Base>        ;
     [add]  --> <Base> + <Base> ;
     [sub]  --> <Base> - <Base> ;
     [mul]  --> <Base> * <Base> ;
     [div]  --> <Base> / <Base> ;

  Base[num] --> <Lex.Num> ;
      [var] --> <Lex.Var> ;
}