*     <dd>last released minor</dd>
 * </dl>
 *
 * <ul>
 * <li>the unreleased <b>major</b>, M+1.0.0 on the `master` branch</li>
 * <li>the unreleased <b>minor</b>,  M.N.0 on the `M.x` (x is literal) branch</li>
 * <li>the unreleased <b>bugfix</b>, M.N.c (c &gt; 0) on the `M.N` branch</li>
 * <li>the unreleased <b>maintenance</b>, M-1.d.e ( d &gt; 0, e &gt; 0) on the `(M-1).d` branch</li>
 * </ul>

  }

  /**
   * Verifies that a collection is immutable.
   *
   * <p>A collection is considered immutable if:
   *
   * <ol>
   *   <li>All its mutation methods result in UnsupportedOperationException, and do not change the
   *       underlying contents.
   *   <li>All methods that return objects that can indirectly mutate the collection throw
   *       UnsupportedOperationException when those mutators are called.
   * </ol>

  /**
   * Determines whether two possibly-null objects are equal. Returns:
   *
   * <ul>
   *   <li>{@code true} if {@code a} and {@code b} are both null.
   *   <li>{@code true} if {@code a} and {@code b} are both non-null and they are equal according to
   *       {@link Object#equals(Object)}.
   *   <li>{@code false} in all other situations.
   * </ul>
   *

 * second invocation, so implementors have made various choices, including:
 *
 * <ul>
 *   <li>returning the same iterator again
 *   <li>throwing an exception of some kind
 *   <li>or the usual, <i>robust</i> behavior, which all known {@link Collection} implementations
 *       have, of returning a new, independent iterator
 * </ul>
 *

 *
 * <p>Supported values:</p>
 * <ul>
 *   <li>{@code hc4} - Use Apache HttpComponents 4.x ({@link Hc4HttpClient})</li>
 *   <li>{@code hc5} or not set - Use Apache HttpComponents 5.x ({@link Hc5HttpClient}) - default</li>
 * </ul>
 *
 * <p>This class extends {@link FaultTolerantClient} to provide automatic retry functionality

 * Benchmarks (as of December 2011) show that:
 *
 * <ul>
 *   <li>for an unnested {@code lock()} and {@code unlock()}, a cycle detecting lock takes 38ns as
 *       opposed to the 24ns taken by a plain lock.
 *   <li>for nested locking, the cost increases with the depth of the nesting:
 *       <ul>
 *         <li>2 levels: average of 64ns per lock()/unlock()
 *         <li>3 levels: average of 77ns per lock()/unlock()

   * does not include "was cancelled." To be explicit: If this method returns a non-null value,
   * then:
   *
   * <ul>
   *   <li>{@code isDone()} must return {@code true}
   *   <li>{@code isCancelled()} must return {@code false}
   *   <li>{@code get()} must not block, and it must throw an {@code ExecutionException} with the
   *       return value of this method as its cause
   * </ul>
   *

 * <ol>
 *   <li>Initialization: Sets up the crawler context and initializes the URL filter.</li>
 *   <li>Thread Creation: Creates a group of crawler threads to perform the actual crawling.</li>
 *   <li>Execution: Starts the crawler threads and waits for them to complete.</li>
 *   <li>Cleanup: Deletes the crawled data and clears the URL filter.</li>
 * </ol>
 *

     * <ul>
     * <li>{@code *} (since 2.0.5)= everything,</li>
     * <li>{@code external:*}  (since 2.0.9)= everything not on the localhost and not file based,</li>
     * <li>{@code external:http:*} (since 3.8.0)= any repository not on the localhost using HTTP,</li>
     * <li>{@code repo,repo1}  (since 2.0.9)= {@code repo} or {@code repo1},</li>

     * <li>The parent uses aggregation and refers to the child via the modules section</li>
     * <li>The module path to the child is considered to
     * point at the POM rather than its base directory if the path ends with ".xml" (ignoring case)</li>
     * <li>The name of the child's base directory matches the artifact id of the child.</li>
     * </ul>