The example defines a subtype which represents a file on disk. Any parameter conforming to that subtype must name a file that is present on disk. Think of thi

The example defines a subtype which represents a file on disk. Any parameter conforming to that subtype must name a file that is present on disk. Think of this subtype as a named precondition -- it's a concise, intention-revealing way of reporting error conditions without cluttering the internals of the function with duplicate error-handling code.