带引号和不带引号

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 ")"。

产生式里 "let"、";" 带引号，IDENT、expr 不带。区别是【0】。