!! +var+ +exec+ +string+
 $lst_tit product$lst_titend$type_both
 $lst_alias prod, multiply
  $(lst_syntax)product $emph fn$emphend for $emph v$emphend=$emph v1$emphend
  to $emph v2$emphend [step $emph st $emphend], 或 <br/>
$ !product $emph fn$emphend for $emph v$emphend in 
  $emph v1,v2,v3,...$emphend</tt>
  $lst_mean 计算变量 $emph v $emphend 逐个被 $emph v1$emphend 到
  $emph v2$emphend 的值替换后函数 $emph fn(v)$emphend 的乘积. <p>
  在第二句里, 替换的值是表 $emph v1,v2,...$emphend 里的各项.

