code: public static Key encodeKey(String bucket, String key) {
      Key res = encodeKeyImpl(bucket, key);
  //    assert checkBijection(res, bucket, key);
      return res;
    }
docs: Creates the key for given S3 bucket and key. Returns the H2O key, or null if the key cannot be
  created.
  
  @param bucket
  Bucket name
  @param key
  Key name (S3)
  @return H2O key pointing to the given bucket and key.