Root element of pipeline

I know this sounds dumb ... but I cant find this anywhere in the specs or 
even in the non-specs.
Is the root element of every XPROC document a <p:pipeline> element ?
Or is there cases where other elements could be considered valid roots ?
( ignoring the <p:library> element for now )

Thanks (especially for any pointers to the docs where this is obvious)

-David
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David A. Lee
dlee@calldei.com
http://www.calldei.com