code: public Map<List<String>, Double> extractNestedAmounts(XEvent event) {
  		return XCostAmount.instance().extractNestedValues(event);
  	}
docs: Retrieves a map containing all cost amounts for all descending attributes
  of an event.
  
  For example, the XES fragment:
  
  <pre>
  {@code
  <event>
  <string key="a" value="">
  <float key="cost:amount" value="10.00"/>
  <string key="b" value="">
  <float key="cost:amount" value="20.00"/>
  </string>
  <string key="c" value="">
  <float key="cost:amount" value="30.00"/>
  </string>
  </string>
  <string key="b" value="">
  <float key="cost:amount" value="15.00"/>
  </string>
  <string key="c" value="">
  <float key="cost:amount" value="25.00"/>
  </string>
  </event>
  }
  </pre>
  
  should result into the following:
  
  <pre>
  [[[a] 10.00] [[a b] 20.00] [[a c] 30.00] [[b] 15.00] [[c] 25.00]]
  </pre>
  
  @param event
  Event to retrieve all cost amounts for.
  @return Map from all descending keys to cost amounts.