expr: _additive ;

_additive: sum | difference | _multiplicative ;
sum: _additive '+' _multiplicative ;
difference: _additive '-' _multiplicative ;

_multiplicative: product | quotient | _primary ;
product: _multiplicative '*' _primary ;
quotient: _multiplicative '/' _primary ;

_primary: NUMBER
       | brackets
       | function ;
brackets: '(' _additive ')' ;
function: SYMBOL '(' _additive ')' ;