译自：http://freemarker.org/docs/pgui_datamodel_directive.html#autoid_37
例子：
输入：
foo
<@upper>
bar
<#-- All kind of FTL is allowed here -->
<#list ["red", "green", "blue"] as color>
${color}
</#list>
baaz
</@upper>
wombat输出：
foo
BAR
RED
GREEN
BLUE
BAAZ
wombat此处还有自定义指令的内容，详情参照官网说明。