I note the following requirement: BidirectionalSeekableDevice Refinement of Device with mode convertible to bidirectional_seekable That seems wrong to me. Shouldn't it be Refinement of Device with mode convertible to both bidirectional and to seekable ? Or is a device that is both seekable and bidirectional not always also bidirectional_seekable? Is that some further refinement? -- Dave Abrahams Boost Consulting www.boost-consulting.com