code: func (f *MockFile) Read(b []byte) (n int, err error) {
  	if f.fs.WithReadError {
  		return 0, readErr
  	}
  
  	count := len(b)
  	if len(f.Contents) < count {
  		count = len(f.Contents)
  	}
  	copy(b, []byte(f.Contents)[:count])
  	f.Seq = f.fs.next()
  
  	return count, nil
  }
docs: // Read copies b bytes from f.Contents