return Optional.ofNullable(parserRequest().stdIn());
    }

    /**
     * Returns the output stream for the Maven execution, if running in embedded mode.
     *
     * @return an {@link Optional} containing the output stream, or empty if not applicable
     */
    @Nonnull
    default Optional<OutputStream> stdOut() {
        return Optional.ofNullable(parserRequest().stdOut());
    }

            public Optional<Source> getInstallationSettingsSource() {
                return Optional.ofNullable(installationSettingsSource);
            }

            @Nonnull
            @Override
            public Optional<Source> getProjectSettingsSource() {
                return Optional.ofNullable(projectSettingsSource);
            }

            @Nonnull
            @Override

     *
     * @param <T> the type of the entity
     * @param entity the entity (can be null)
     * @return the OptionalEntity
     */
    public static <T> OptionalEntity<T> ofNullable(final T entity) {
        return OptionalEntity.ofNullable(entity, () -> {});
    }

            public Optional<Source> getInstallationToolchainsSource() {
                return Optional.ofNullable(installationToolchainsSource);
            }

            @Nonnull
            @Override
            public Optional<Source> getUserToolchainsSource() {
                return Optional.ofNullable(userToolchainsSource);
            }

            @Override
            public boolean equals(Object o) {

          throw multiples(true);
        }
        return this;
      }
    }

    Optional<Object> getOptional() {
      if (extras.isEmpty()) {
        return Optional.ofNullable(element);
      } else {
        throw multiples(false);
      }
    }

    Object getElement() {
      if (element == null) {
        throw new NoSuchElementException();

      }
    }

    @IgnoreJRERequirement // see enclosing class (whose annotation Animal Sniffer ignores here...)
    Optional<Object> getOptional() {
      if (extras.isEmpty()) {
        return Optional.ofNullable(element);
      } else {
        throw multiples(false);
      }
    }

    Object getElement() {
      if (element == null) {
        throw new NoSuchElementException();

                @Override
                public Optional<org.apache.maven.api.Toolchain> createDefaultToolchain()
                        throws ToolchainFactoryException {
                    return Optional.ofNullable(v3Factory.createDefaultToolchain())
                            .map(ToolchainManagerFactory.this::getToolchainV4);
                }
            });
        }
        allFactories.putAll(v4Factories);

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