def substitution_value(index)
      sha = Digest::SHA1.digest(index.to_s)
      Base64.urlsafe_encode64(sha).gsub(/[^A-Za-z]/, '')[0..5]
    end
def substitution_value(index)
      sha = Digest::SHA1.digest(index.to_s)
      Base64.urlsafe_encode64(sha).gsub(/[^A-Za-z]/, '')[0..5]
    end
Generate a repeatable 5 character uniform alphabetical nonsense string
 to allow parsing as a URI