Contributions are of the form:
<source name="..." before="..." after="..." class="..." service-id="..."/>
Sources are ordering based on the name, before and after elements. before and after may be comma-seperated lists of other sources, may be the simple value *, or may be omitted.
Only one of class and service-id attributes should be specified. The former is the complete name of a class (implementing the SymbolSource interface). The second is used to contribute a service (which must also implement the interface).