code: function preferredProviderFor (uri) {
    // First, determine if the uri is an OIDC provider
    return providerExists(uri)
      .then(providerUri => {
        if (providerUri) {
          return providerUri // the given uri's origin hosts an OIDC provider
        }
  
        // Given uri is not a provider (for example, a static Web ID profile URI)
        // Discover its preferred provider
        return discoverProviderFor(uri)
      })
  }
docs: @param uri {string} Provider URI or Web ID URI
  
  @returns {Promise<string>}