code: func (f *FilterHeaderStore) FetchHeader(hash *chainhash.Hash) (*chainhash.Hash, error) {
  	// Lock store for read.
  	f.mtx.RLock()
  	defer f.mtx.RUnlock()
  
  	height, err := f.heightFromHash(hash)
  	if err != nil {
  		return nil, err
  	}
  
  	return f.readHeader(height)
  }
docs: // FetchHeader returns the filter header that corresponds to the passed block
  // height.