    return 'E';
&lt;&lt;EOF&gt;&gt;               return 'EOF';

/lex

/* operator associations and precedence */

%left '+' '-'
%left '*' '/'
%left '^'
%left UMINUS

%start expressions

%% /* language grammar */

expressions
: e EOF
{print($1); return $1;}
;

e
: e '+' e
{$$ = $1+$3;}
| e '-' e
{$$ = $1-$3;}
| e '*' e
{$$ = $1*$3;}
| e '/' e
{$$ = $1/$3;}
| e '^' e
{$$ = Math.pow($1, $3);}
| '-' e %prec UMINUS
{$$ = -$2;}
| '(' e ')'
{$$ = $2;}
| NUMBER
{$$ = Number(yytext);}
| E
{$$ = Math.E;}
| PI
{$$ = 