class OOCalcLinesServlet serves requests like

GET /OOCalcGradesheetDocs/sheet_id/?id=123456789 HTTP/1.1 From: myhost.mysubdomain.mydomain.com

• if ?id=… not specified, replies with 412 (Precondition failed)
• if no row is found for the id, replies with 404 (Not Found)
• else replies with 200 (OK) and the payload from the students_html message passed to the SpreadsheetContent object for sheet_id.

Requests other than GET are not served!