<class person first=req last=req age=req/>
<vector <person "Joe" "S" 21/> <person "Mary" "S" 23/> <person "Mike" "S" 34/> />.
    <group> .first.<subvector end=1/> .age </>.<to_htm/>

Instances of person first last age J first 21 age Joe S 21 M first 23 age Mary S 23 34 age Mike S 34

<class event name=required=string on=required=datetime/>
<doc on=datetime.to_quarters>
   Converts a datetime into a datetime with a day of Jan 1, Apr 1, Jul 1, or Oct 1.
 
   <test
    result=<datetime 2004 4 1 0 0 0 0/>
    <datetime 2004 6 10/>.<to_quarters/>
   />
  </doc>
<method datetime.to_quarters>
   <if> .month.<less_or_equal 3/> 1
        .month.<less_or_equal 6/> 2
        .month.<less_or_equal 9/> 3
        else                      4
   </if>
  </method>
<vector
   <event "event1" <datetime 2004 2 10/> />
   <event "event1" <datetime 2004 1 13/> />
   <event "event2" <datetime 2004 5 19/> /> 
   <event "event3" <datetime 2004 6 12/> />
  />.<group 
    group_by=<v> <method by_year>    .on.year </> 
                 <method by_quarter> .on.<to_quarters/> </> 
             </v>
  />.<to_htm/>

Instances of event name on 1 Q event1 Feb 10, 2004 event1 Jan 13, 2004 2 Q event2 May 19, 2004 event3 Jun 12, 2004