code: public Pac4j unauthenticated(Supplier<UserProfile> provider) {
      requireNonNull(provider, "Unauthenticated provider required.");
      return unauthenticated(req -> provider.get());
    }
docs: Set a default action which is execute when no user is logged in.
  
  <pre>{@code
  {
  use(new Pac4j()
  .unauthenticated(() -> {
  UserProfile anonymous = ...
  return anonymous;
  })
  );
  
  get("/", () -> {
  // might or might not be anonymous
  UserProfile profile = require(UserProfile.class);
  return ...;
  }
  }
  }</pre>
  
  The default action throws a <code>403</code> error.
  
  
  @param provider Unauthenticated user provider.
  @return This module.