括号怎么改变结合

玩具语言的文法（八节共用）：program := stmt*；stmt := "let" IDENT "=" expr ";" | IDENT "=" expr ";" | "if" "(" expr ")" block ("else" block)? | "while" "(" expr ")" block；block := "{" stmt* "}"；expr := comparison；comparison := sum ((">=" | "<=" | "==" | "!=" | "<" | ">") sum)?；sum := term (("+" | "-") term)*；term := factor (("*" | "/") factor)*；factor := NUMBER | STRING | IDENT | "(" expr ")"。

( 1 + 2 ) * 3 里，括号让加法先于乘法。parser 做到这一点靠的是【0】。