-func NewSessionV0(ws wsConn, sendq chan<- interface{}, db *sql.DB, pc permChecker) (session, error) {
+// newSessionV0 returns a v0 session: a partial port of the Rails/puma
+// implementation, with just enough functionality to support Workbench
+// and arv-mount.
+func newSessionV0(ws wsConn, sendq chan<- interface{}, db *sql.DB, pc permChecker, ac *arvados.Client) (session, error) {