<idea-plugin>
  <id>com.intellij.myPlugin</id>

  <extensionPoints>
    <extensionPoint qualifiedName="com.intellij.myPlugin.ext" interface="java.lang.Runnable"/>
  </extensionPoints>

  <extensions defaultExtensionNs="com.intellij.myPlugin">
    <ext implementation="foo.MyRunnable"/>
  </extensions>

</idea-plugin>