!! +var+ +exec+ +string+
 $lst_tit sum$lst_titend$type_both
 $lst_alias add
  $(lst_syntax)sum $emph fn$emphend for $emph v$emphend=$emph v1$emphend
  to $emph v2$emphend [step $emph st $emphend], 或 <br/>
$ !sum $emph fn$emphend for $emph v$emphend in 
  $emph v1,v2,v3,...$emphend</tt> <br/>
  $lst_mean 把变量 $emph v $emphend 用 $emph v1$emphend 与
  $emph v2$emphend 之间的值逐次替换后, 计算函数 $emph fn(v)$emphend 值之和. <p>
  在第二种情形, 用表 $emph v1,v2,...$emphend 里的值逐次替换.
