code: def double_sha256(ba):
      
      d1 = hashlib.sha256(ba)
      d2 = hashlib.sha256()
      d1.hexdigest()
      d2.update(d1.digest())
      return d2.hexdigest()
docs: Perform two SHA256 operations on the input.
  
      Args:
          ba (bytes): data to hash.
  
      Returns:
          str: hash as a double digit hex string.