*/
    static Function<String, String> memoize(Function<String, String> callback) {
        Map<String, Optional<String>> cache = new HashMap<>();
        return s -> cache.computeIfAbsent(s, v -> Optional.ofNullable(callback.apply(v)))
                .orElse(null);
    }

        if (artifact == null) {
            return null;
        }

        Artifact result = toArtifact(artifact);

        List<Exclusion> excl = Optional.ofNullable(exclusions).orElse(Collections.emptyList()).stream()
                .map(RepositoryUtils::toExclusion)
                .collect(Collectors.toList());

        return project.getRootDirectory();
    }

    @Override
    public Optional<Project> getParent() {
        MavenProject parent = project.getParent();
        return Optional.ofNullable(session.getProject(parent));
    }

    @Nonnull
    private DependencyCoordinates toDependency(org.apache.maven.api.model.Dependency dependency) {
        return new DependencyCoordinates() {

     *
     * @return the {@code javax.tool} enumeration value corresponding to this {@code JavaPathType}
     */
    public Optional<JavaFileManager.Location> location() {
        return Optional.ofNullable(location);
    }

    /**
     * Returns the path type associated to the given {@code javax.tool} location.
     * This method is the converse of {@link #location()}.
     *

        // Session keep tracks of the Project per project id,
        // so we cannot use session.getProject(p) for forked projects
        // which are temporary clones
        return Optional.ofNullable(getMavenProject(project).getExecutionProject())
                .map(p -> new DefaultProject(session, p));
    }

    private MavenProject getMavenProject(Project project) {

   * reference; otherwise returns {@link Optional#absent}.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> this method is equivalent to Java 8's
   * {@code Optional.ofNullable}.
   */
  public static <T> Optional<T> fromNullable(@CheckForNull T nullableReference) {
    return (nullableReference == null) ? Optional.<T>absent() : new Present<T>(nullableReference);
  }

  /**

        return incidentNodes(edge).adjacentNode(nodePresent).equals(nodeToCheck);
      }
    };
  }

  @Override
  public Optional<E> edgeConnecting(N nodeU, N nodeV) {
    return Optional.ofNullable(edgeConnectingOrNull(nodeU, nodeV));
  }

  @Override
  public Optional<E> edgeConnecting(EndpointPair<N> endpoints) {
    validateEndpoints(endpoints);

                private final Map<Artifact, Path> paths = new ConcurrentHashMap<>();

                @Override
                public Optional<Path> getPath(Artifact artifact) {
                    return Optional.ofNullable(paths.get(artifact));
                }

                @Override
                public void setPath(ProducedArtifact artifact, Path path) {
                    paths.put(artifact, path);
                }

        }
        final long execTime = systemHelper.getCurrentTimeAsLong() - startTime;

        return searchResult.build(requestBuilder, execTime, OptionalEntity.ofNullable(response, () -> {}));
    }

    public <T> T search(final String index, final SearchCondition<SearchRequestBuilder> condition,
            final SearchResult<T, SearchRequestBuilder, SearchResponse> searchResult) {

                Supplier<? extends Activation.Builder> creator, Activation.Builder builder, Activation target) {
            stk.push(nextFrame("file", Activation::getFile));
            Optional.ofNullable(target.getFile());
            try {
                return super.transformActivation_File(creator, builder, target);
            } finally {
                stk.pop();
            }
        }