docs: def to_ipv6(key):
      """Get IPv6 address from a public key."""
  
      if key[-2:] != '.k':
          raise ValueError('Key does not end with .k')
  
      key_bytes = base32.decode(key[:-2])
      hash_one = sha512(key_bytes).digest()
      hash_two = sha512(hash_one).hexdigest()
  
      return ':'.join([hash_two[i:i+4] for i in range(0, 32, 4)])