code: func (d *definition) readFrom(r io.Reader) error {
  	err := binary.Read(r, binary.LittleEndian, d)
  	if err != nil {
  		return err
  	}
  	magic := reflect.TypeOf(*d).Field(0).Tag.Get("is")
  	if !bytes.Equal(d.Magic[:], []byte(magic)) {
  		return fmt.Errorf("cram: not a cram file: magic bytes %q", d.Magic)
  	}
  	return nil
  }
docs: // readFrom populates a definition from the given io.Reader. If the magic
  // number of the file is not "CRAM" readFrom returns an error.