code: func (f *lightFetcher) requestAmount(p *peer, n *fetcherTreeNode) uint64 {
  	amount := uint64(0)
  	nn := n
  	for nn != nil && !f.checkKnownNode(p, nn) {
  		nn = nn.parent
  		amount++
  	}
  	if nn == nil {
  		amount = n.number
  	}
  	return amount
  }
docs: // requestAmount calculates the amount of headers to be downloaded starting
  // from a certain head backwards