* use the {@code HashMap} constructor directly, taking advantage of <a
   * href="https://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html#type-inference-instantiation">"diamond"
   * syntax</a>.
   *
   * @return a new, empty {@code HashMap}
   */
  @SuppressWarnings("NonApiType") // acts as a direct substitute for a constructor call

   * |      |
   * |      |
   * e ---- c ---- f
   * }
   */
  private static final SuccessorsFunction<Character> JAVADOC_GRAPH =
      createUndirectedGraph("ba", "ad", "be", "ac", "ec", "cf");

  /**
   * A diamond shaped directed graph (arrows going down):
   *
   * {@snippet :
   *   a
   *  / \
   * b   c
   *  \ /
   *   d
   * }
   */
  private static final SuccessorsFunction<Character> DIAMOND_GRAPH =

                <jdk>
                  <version>11</version>
                </jdk>
              </toolchains>
            </configuration>
          </plugin>
          <plugin>
            <groupId>com.diamondq.maven</groupId>
            <artifactId>javahome-resolver-maven-plugin</artifactId>
            <version>${javahome-resolver-maven-plugin.version}</version>
            <executions>
              <execution>

                <jdk>
                  <version>11</version>
                </jdk>
              </toolchains>
            </configuration>
          </plugin>
          <plugin>
            <groupId>com.diamondq.maven</groupId>
            <artifactId>javahome-resolver-maven-plugin</artifactId>
            <version>${javahome-resolver-maven-plugin.version}</version>
            <executions>
              <execution>