Currently, elements of a macrodef cannot have default values,
even though attributes can have them.

It would be nice if I can write something like:

<macrodef name="foo">
  <element name="a" optional="true">
    <some />
    <tasks />
  </element>
  <sequential> ... </sequential>
</macrodef>

so that the following two invocations would be equivalent:

<foo>
  <a>
    <some />
    <tasks />
  </a>
</foo>

<foo />  <!-- the nested 'a' element defaulted -->