乘法为什么先于加法

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 ")"。

parser 里没有一行写着「乘先于加」，1 + 2 * 3 却解析成 1+(2*3)。原因是【0】。