code: func (d *ulReqDecoder) decodeDeepen() stateFn {
  	if bytes.HasPrefix(d.line, deepenCommits) {
  		return d.decodeDeepenCommits
  	}
  
  	if bytes.HasPrefix(d.line, deepenSince) {
  		return d.decodeDeepenSince
  	}
  
  	if bytes.HasPrefix(d.line, deepenReference) {
  		return d.decodeDeepenReference
  	}
  
  	if len(d.line) == 0 {
  		return nil
  	}
  
  	d.error("unexpected deepen specification: %q", d.line)
  	return nil
  }
docs: // Expected format: deepen <n> / deepen-since <ul> / deepen-not <ref>