def get(req)
    target_uri = req.header.request_uri
    return nil unless @challengeable.find { |uri, ok|
      Util.uri_part_of(target_uri, uri) and ok
    }
    return @cred if @cred
    Util.hash_find_value(@auth) { |uri, cred|
      Util.uri_part_of(target_uri, uri)
    }
  end
def get(req)
    target_uri = req.header.request_uri
    return nil unless @challengeable.find { |uri, ok|
      Util.uri_part_of(target_uri, uri) and ok
    }
    return @cred if @cred
    Util.hash_find_value(@auth) { |uri, cred|
      Util.uri_part_of(target_uri, uri)
    }
  end
send cred only when a given uri is;
   - child page of challengeable(got WWW-Authenticate before) uri and,
   - child page of defined credential