syntax_ids:
add_expr
sub_expr
mul_expr
div_expr
number_expr
group_expr
term
expr

rules:

expr
: expr plus term
=> new_node add_expr $0 $1 $2
: expr minus term
=> new_node sub_expr $0 $1 $2
: term
=> new_node expr $0
;

term
: term multiply factor
=> new_node mul_expr $0 $1 $2
: term divide factor
=> new_node div_expr $0 $1 $2
: factor
=> new_node term $0
;

factor
: number
=> new_node number_expr $0
: lparen expr rparen
=> new_node group_expr $1
;