%token number
%token eol
%token opplus
%token opminus
%token optimes
%token opdiv
%token braceopen
%token braceclose
%%
LINES   : LINES EXPR eol    { print "Result: ", $2, "\n";
                                  $$ = $2; }
    | LINES eol
    |
    ;

EXPR    : EXPR opplus  TERM { $$ = $1 + $3; }
    | EXPR opminus TERM { $$ = $1 - $3; }
    | TERM
    ;

TERM    : TERM optimes FACTOR   { $$ = $1 * $3; }
    | TERM opdiv   FACTOR   { $$ = $3 == 0 ? "undef" : $1/$3; }
    | FACTOR
    ;

FACTOR  : braceopen EXPR braceclose   { $$ = $2; }
    | number
    ;
%%