accessing a map value by key in a variable

<?xml version="1.0" encoding="UTF-8"?>
<?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit" arg0="./mywin"?>
<window id="mywin" border="none">
ZK name check list :
	<zscript>
  // prepare persons of one type
  Set persons = new HashSet();
	HashMap person1 = new HashMap();
	person1.put("firstName", "mary");
	person1.put("lastName", "bearss");
	persons.add(person1);
	HashMap person2 = new HashMap();
	person2.put("firstName", "ann");
	person2.put("lastName", "moe");
	persons.add(person2);
	HashMap person3 = new HashMap();
	person3.put("firstName", "arnold");
	person3.put("lastName", "schwartz");
	persons.add(person3);
	HashMap person4 = new HashMap();
	person4.put("firstName", "pete");
	person4.put("lastName", "boyle");
	persons.add(person4);

  //make a map of persons of several types
  Map personTypes = new HashMap();
  personTypes.put("teachers", persons);

  String type = "teachers";

  </zscript>

	<listbox model="@{personTypes.teachers}" selectedItem="@{selected}" rows="4">
		<listhead >
				<listheader label="First Name"/>
				<listheader label="Last Name"/>
		</listhead>
		<listitem self="@{each=person}">
      <listcell label="@{person.firstName}" />
      <listcell label="@{person.lastName}" />
    </listitem>
	</listbox>
	<grid>
		<columns>
			<column label="First Name" />
			<column label="Last Name" />
		</columns>
		<rows>
			<row>
				<textbox value="@{selected.firstName}" />
				<textbox value="@{selected.lastName}" />
			</row>
		</rows>
	</grid>
</window>

<listbox model="@{personTypes [ type ] }" selectedItem="@{selected}" rows="4">