text: def make_block_number_to_hash_lookup_key(block_number):
      number_to_hash_key = b'block-number-to-hash:%d' % block_number
      return number_to_hash_key
  
  
  def make_block_hash_to_score_lookup_key(block_hash):
      return b'block-hash-to-score:%s' % block_hash
  
  
  def make_transaction_hash_to_data_lookup_key(transaction_hash):
      '''
      Look up a transaction that is pending, after being issued locally
      '''
      return b'transaction-hash-to-data:%s' % transaction_hash
  
  
  def make_transaction_hash_to_block_lookup_key(transaction_hash):
      return b'transaction-hash-to-block:%s' % transaction_hash
  
  
  def get_parent_header(block_header, db):
      """
      Returns the header for the parent block.
      """
      return db.get_block_header_by_hash(block_header.parent_hash)
  
  
  def get_block_header_by_hash(block_hash, db):
      """
      Returns the header for the parent block.
      """
      return db.get_block_header_by_hash(block_hash)
  