I'm guessing Pete Daoust is thinking in the context of his business where he has a PO, and, or, a signed Invoice. Also, I am guessing he can prove he provided a service or product.
Printed copy · Anthology Volume 3, page 291Printed copy from The Tender For Law Anthology, Volume 3, page 291 (printed 13 June 2014). Added by this site; not part of the Facebook export.