Invocation
==========
fsexpr collect [--one-device] --out OUT PATH
  Scan files under PATH (skipping other devices if --one-device is
  provided) and write a file system map into OUT

fsexpr dump [--expr EXPR] MAP1 MAP2 ... MAPN
  Load the file system maps MAP1, ..., MAPN and list their entries,
  retricting the results to the expression EXPR if provided.

fsexpr examine [--no-history] MAP1 MAP2 ... MAPN
  Load the file system maps and run an interactive CLI
  Unless the --no-history option is present, command-line history
  will be saved to ~/.fsmap-hist