code: func NewFilesystemBetweenPaths(readPath, writePath string) *Filesystem {
  	return &Filesystem{
  		path:     writePath,
  		readPath: readPath,
  	}
  }
docs: // NewFilesystemBetweenPaths creates a filesystem-based state manager that
  // reads an initial snapshot from readPath and then writes all new snapshots to
  // writePath.