WASHINGTON - The Bureau of Settled Theorems said Saturday that a completion may not file as a proof until a clerk stamps the Lean file at the window. Informal write-ups are returned as drafts. Ordinary chat may continue without a stamp. Under Circular ST-2, a theorem is a stamped Lean file. The user presents the file at the window, where a clerk inks SETTLED on the first page. The completion may issue only after that page is date-stamped. "A proof is the stamp," said Clara Venn, counsel to the bureau. "We do not file informal." Labs that publish journal drafts may keep those notes. Runs that produce a theorem without a stamped Lean file are sent back incomplete. A staffer at one research desk, not authorized to speak, said the mailroom now holds stacks of unstamped printouts from the morning filing. The rule takes effect 18 September for all duty windows that issue proofs. The bureau said it will publish a specimen Lean page after the first week of filings.