Contributions are of the form:
<source name="..." before="..." after="..." class="..." service-id="..."/>
Sources are ordering based on the
may be comma-seperated lists of other sources, may be the simple value
*, or may be omitted.
Only one of
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).