  We present the most interesting elements of the correctness specification of
BilbyFs, a performant Linux flash file system. The BilbyFs specification
supports asynchronous writes, a feature that has been overlooked by several
file system verification projects, and has been used to verify the correctness
of BilbyFs's fsync() C implementation. It makes use of nondeterminism to be
concise and is shallowly-embedded in higher-order logic.
