public data TYPE =
	  natural() | string();
	
public alias PicoId = str;
	
public data PROGRAM =
  program(list[DECL] decls, list[STATEMENT] stats);

public data DECL =
  decl(PicoId name, TYPE tp);

public data EXP =
      id(PicoId name)
    | natCon(int iVal)
    | strCon(str sVal)
    | add(EXP left, EXP right)
    | sub(EXP left, EXP right)
    | conc(EXP left, EXP right)
    ;

public data STATEMENT =
  asgStat(PicoId name, EXP exp)
| ifElseStat(EXP exp, list[STATEMENT] thenpart, list[STATEMENT] elsepart)
| ifThenStat(EXP exp, list[STATEMENT] thenpart)
| whileStat(EXP exp, list[STATEMENT] body)
| doUntilStat(EXP exp, list[STATEMENT] body)
| unlessStat(EXP exp, list[STATEMENT] body)
;