# 约束 whisper 只输出「乘法口诀答案」：可选(两因数[得]) + 一个中文数字
# 只允许中文数字：阿拉伯数字会把「十四/七十四」压扁成 14/74，丢掉「十」的信息
# alias 是发音兼容别名（n/l 不分、口语、同音），前端 DIG 表映射回数字
root    ::= prefix? product
prefix  ::= fd fd "得"?
fd      ::= "一" | "二" | "三" | "四" | "五" | "六" | "七" | "八" | "九"
product ::= tens | teen | unit | "十"
unit    ::= "零" | "一" | "二" | "三" | "四" | "五" | "六" | "七" | "八" | "九" | alias
alias   ::= "牛" | "流" | "柳" | "溜" | "两" | "酒"
teen    ::= "十" unit
tens    ::= unit "十" unit?