code: func (p *Package) Copy(name string, r io.Reader) {
  	if p.err != nil {
  		return
  	}
  
  	zf, err := p.z.Create(name)
  	if err != nil {
  		p.err = err
  		return
  	}
  
  	checksum, err := copyAndChecksum(zf, r)
  	if err != nil {
  		p.err = err
  		return
  	}
  
  	p.manifest[name] = checksum
  }
docs: // Copy reader to the push package.