code: func NewFeeEstimator(maxRollback, minRegisteredBlocks uint32) *FeeEstimator {
  	return &FeeEstimator{
  		maxRollback:         maxRollback,
  		minRegisteredBlocks: minRegisteredBlocks,
  		lastKnownHeight:     mining.UnminedHeight,
  		binSize:             estimateFeeBinSize,
  		maxReplacements:     estimateFeeMaxReplacements,
  		observed:            make(map[chainhash.Hash]*observedTransaction),
  		dropped:             make([]*registeredBlock, 0, maxRollback),
  	}
  }
docs: // NewFeeEstimator creates a FeeEstimator for which at most maxRollback blocks
  // can be unregistered and which returns an error unless minRegisteredBlocks
  // have been registered with it.