!! +exec+ +vardef+
$lst_tit default$lst_titend$type_exec
  $(lst_syntax)default $(emph)name$emphend=$(emph)value$emphend</tt>
  $lst_mean 如果变量 $(emph)name$emphend 还没有定义, 则把 $(emph)value$emphend
  赋值给 $(emph)name$emphend. <br/>
  $(emph)value$emphend 可以是变量命令(带着可选参数).
