Mmhh... what if other parameters are also URIs?
If src attribute to be interpreted always as a Source object... We could add map:source as well:
<map:transformer type="three-source-transformer">
<map:source name="first" value=""/>
<map:source name="second" value=""/>
<map:source name="third" value=""/>
<map:parameter name="foo" value="bar"/>
...
</map:transformer>Vadim
