code: func (f *MtimeFS) save(name string, real, virtual time.Time) {
  	if f.caseInsensitive {
  		name = UnicodeLowercase(name)
  	}
  
  	if real.Equal(virtual) {
  		// If the virtual time and the real on disk time are equal we don't
  		// need to store anything.
  		f.db.Delete(name)
  		return
  	}
  
  	mtime := dbMtime{
  		real:    real,
  		virtual: virtual,
  	}
  	bs, _ := mtime.Marshal() // Can't fail
  	f.db.PutBytes(name, bs)
  }
docs: // "real" is the on disk timestamp
  // "virtual" is what want the timestamp to be