Example: find all services that are powered by an energy provider complying
  with any level of the given energy policies and offering any type of regional
  policy.

// filtering continues until the search result is reduced to the number of
// desired maximum hits. filtering is terminated with the result of the
// last filter processed as the end result when less than the desired number
// of minimum hits remains.

<?xml version="1.0" encoding="UTF-8"?>
<esp>
  <search>                   // required
    <policyGroup type="environment">
      <policy type="energy" url="http://www.greenpeace-energy.de/2005.1" level="*"/>
      <policy type="energy" url="http://www.lichtblick.de/2005.1" level="*"/>
      <policy type="region" url="*"
    </policyGroup>
  </search>
  <filter>                   // first filter
    <policyGroup type="environment">
      <policy type="energy" url="http://www.greenpeace-energy.de/2005.1" level="C"/>
    </policyGroup>
  </filter>
  <filter>                   // second filter
    <policyGroup type="environment">
      <policy type="energy" url="http://www.greenpeace-energy.de/2005.1" level="B"/>
    </policyGroup>
  </filter>
  <filter>                   // goal
    <policyGroup type="environment">
      <policy type="energy" url="http://www.greenpeace-energy.de/2005.1" level="A"/>
    </policyGroup>
  </filter>
</esp>