*
 * All start/connect/acquire events will eventually receive a matching end/release event, either
 * successful (non-null parameters), or failed (non-null throwable). The first common parameters of
 * each event pair are used to link the event in case of concurrent or repeated events e.g.
 * `dnsStart(call, domainName)` → `dnsEnd(call, domainName, inetAddressList)`.
 *
 * Events are typically nested with this structure:
 *

      if (result == null) result = ArrayList(2)
      result.add(value(i))
    }
  }
  return result?.unmodifiable().orEmpty()
}

internal fun Headers.commonIterator(): Iterator<Pair<String, String>> = Array(size) { name(it) to value(it) }.iterator()

internal fun Headers.commonNewBuilder(): Headers.Builder {
  val result = Headers.Builder()
  result.namesAndValues += namesAndValues
  return result
}

 *       instance of {@code BiMap}.
 *   <dt>{@link Table}
 *   <dd>A new type, which is similar to {@link java.util.Map}, but which indexes its values by an
 *       ordered pair of keys, a row key and column key.
 *   <dt>{@link Multiset}
 *   <dd>An extension of {@link java.util.Collection} that may contain duplicate values like a

 *       instance of {@code BiMap}.
 *   <dt>{@link Table}
 *   <dd>A new type, which is similar to {@link java.util.Map}, but which indexes its values by an
 *       ordered pair of keys, a row key and column key.
 *   <dt>{@link Multiset}
 *   <dd>An extension of {@link java.util.Collection} that may contain duplicate values like a

  /**
   * Returns a new equivalence relation for {@code F} which evaluates equivalence by first applying
   * {@code function} to the argument, then evaluating using {@code this}. That is, for any pair of
   * non-null objects {@code x} and {@code y}, {@code equivalence.onResultOf(function).equivalent(a,
   * b)} is true if and only if {@code equivalence.equivalent(function.apply(a), function.apply(b))}
   * is true.
   *

* Included the...

    }
    for (N node : subgraph.nodes()) {
      for (N successorNode : graph.successors(node)) {
        if (subgraph.nodes().contains(successorNode)) {
          // requireNonNull is safe because the endpoint pair comes from the graph.
          subgraph.putEdgeValue(
              node,
              successorNode,
              requireNonNull(graph.edgeValueOrDefault(node, successorNode, null)));
        }
      }
    }

    }
    for (N node : subgraph.nodes()) {
      for (N successorNode : graph.successors(node)) {
        if (subgraph.nodes().contains(successorNode)) {
          // requireNonNull is safe because the endpoint pair comes from the graph.
          subgraph.putEdgeValue(
              node,
              successorNode,
              requireNonNull(graph.edgeValueOrDefault(node, successorNode, null)));
        }
      }
    }

     *
     * @param css the CSS class name to match
     */
    public void setCss(final String css) {
        this.css = css;
    }

    /**
     * Sets a custom attribute name-value pair that this pruned tag should match.
     *
     * @param name the attribute name to match
     * @param value the attribute value to match
     */
    public void setAttr(final String name, final String value) {

    printEvent("callStart");
  }

  @Override public void callEnd(Call call) {
    printEvent("callEnd");
  }

  ...
}
```

We can use this listener to race a pair of concurrent HTTP requests:

```java
Request washingtonPostRequest = new Request.Builder()
    .url("https://www.washingtonpost.com/")
    .build();
client.newCall(washingtonPostRequest).enqueue(new Callback() {