/** Adds the chain to the seen set, and returns whether all the chain was new to us. */
  private static boolean addCausalChain(Set<Throwable> seen, Throwable param) {
    // Declare a "true" local variable so that the Checker Framework will infer nullness.
    Throwable t = param;

    for (; t != null; t = t.getCause()) {
      boolean firstTimeSeen = seen.add(t);
      if (!firstTimeSeen) {
        /*

        // we want to see all top-level thread groups
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        while (tg.getParent() == null) {
            tg = tg.getParent();
        }

        ThreadGroup[] tgList = new ThreadGroup[tg.activeGroupCount()];
        tg.enumerate(tgList);

        boolean seen = false;

        for (ThreadGroup aTgList : tgList) {

      for (Entry<E> entry : insertionOrder) {
        map.put(entry.getElement(), entry);
      }

      Set<E> seen = new HashSet<>();
      List<Entry<E>> order = new ArrayList<>();
      for (E e : gen.order(new ArrayList<E>(map.keySet()))) {
        if (seen.add(e)) {
          order.add(map.get(e));
        }
      }
      return order;
    }
  }

# Advanced Dependencies

## Parameterized dependencies

All the dependencies we have seen are a fixed function or class.

But there could be cases where you want to be able to set parameters on the dependency, without having to declare many different functions or classes.

Let's imagine that we want to have a dependency that checks if the query parameter `q` contains some fixed content.

But we want to be able to parameterize that fixed content.

# Async Tests

You have already seen how to test your **FastAPI** applications using the provided `TestClient`. Up to now, you have only seen how to write synchronous tests, without using `async` functions.

      for (Entry<E> entry : insertionOrder) {
        map.put(entry.getElement(), entry);
      }

      Set<E> seen = new HashSet<>();
      List<Entry<E>> order = new ArrayList<>();
      for (E e : gen.order(new ArrayList<E>(map.keySet()))) {
        if (seen.add(e)) {
          order.add(map.get(e));
        }
      }
      return order;
    }
  }

   * range [0, k) and ignore the remaining elements.
   */
  private final @Nullable T[] buffer;
  private int bufferSize;

  /**
   * The largest of the lowest k elements we've seen so far relative to this comparator. If
   * bufferSize ≥ k, then we can ignore any elements greater than this value.
   */
  @CheckForNull private T threshold;

   * Sets {@code table[index]} to {@code entry}, where {@code table} is actually a {@code byte[]},
   * {@code short[]}, or {@code int[]}. The value of {@code entry} should fit in the size of the
   * assigned array element, when seen as an unsigned value. So if {@code table} is a {@code byte[]}
   * then we should have {@code 0 ≤ entry ≤ 255}, and if {@code table} is a {@code short[]} then we

   * Sets {@code table[index]} to {@code entry}, where {@code table} is actually a {@code byte[]},
   * {@code short[]}, or {@code int[]}. The value of {@code entry} should fit in the size of the
   * assigned array element, when seen as an unsigned value. So if {@code table} is a {@code byte[]}
   * then we should have {@code 0 ≤ entry ≤ 255}, and if {@code table} is a {@code short[]} then we

# Behind a Proxy

In some situations, you might need to use a **proxy** server like Traefik or Nginx with a configuration that adds an extra path prefix that is not seen by your application.

In these cases you can use `root_path` to configure your application.

The `root_path` is a mechanism provided by the ASGI specification (that FastAPI is built on, through Starlette).

The `root_path` is used to handle these specific cases.