echo "JAVA_HOME is set to \"$JAVA_HOME\", but \"\$JAVA_HOME/bin/java\" does not exist." >&2
      exit 1
    fi
  fi
else
  JAVACMD="`\\unset -f command; \\command -v java`"

  if [ ! -x "$JAVACMD" ] ; then
    echo "The java(1) command does not exist in PATH nor is JAVA_HOME set, so Apache Maven cannot be started." >&2
    exit 1
  fi
fi

    }
}

// in plugin
existingThing = objects.newInstance(DefaultExistingThing.class)
existingThing.convention("some-value")
```

It's not acceptable to treat an unset Provider as if the convention is requested when the convention could be set elsewhere:

```groovy
public interface NewThing {
    Property<String> getSomeProperty()
}

// Unacceptable
String value

   * high 32 bits of each long is the "prev" pointer, whereas the low 32 bits is the "succ" pointer
   * (pointing to the next entry in the linked list). The pointers in [size(), entries.length) are
   * all "null" (UNSET).
   *
   * <p>A node with "prev" pointer equal to {@code ENDPOINT} is the first node in the linked list,
   * and a node with "next" pointer equal to {@code ENDPOINT} is the last node.
   */

     * for the current build session and do not modify the underlying project model.
     *
     * @param project the project to modify
     * @param key they property's key
     * @param value the value or {@code null} to unset the property
     */
    void setProperty(@Nonnull Project project, @Nonnull String key, @Nullable String value);

    /**
     * Returns the original project being built when the input project is a forked project.

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }

  private enum NullEntry implements ReferenceEntry<Object, Object> {
    INSTANCE;

    @Override
    public @Nullable ValueReference<Object, Object> getValueReference() {

	// MINIO_KMS_SECRET_KEY_FILE - either to the argument passed to
	// the container or to a default string (e.g. "minio_master_key").
	//
	// We have to distinguish a explicit config from an implicit. Hence,
	// we unset the env. vars if they are set but empty or contain a path
	// which does not exist. The downside of this check is that if
	// MINIO_KMS_SECRET_KEY_FILE is set to a path that does not exist,

// the file it describes, it may be necessary to modify the Name field
// of the returned header to provide the full path name of the file.
// If compression is desired, callers should set the FileHeader.Method
// field; it is unset by default.
func FileInfoHeader(fi fs.FileInfo) (*FileHeader, error) {
	size := fi.Size()
	fh := &FileHeader{
		Name:               fi.Name(),
		UncompressedSize64: uint64(size),
	}
	fh.SetModTime(fi.ModTime())

### Use the `response_model_exclude_unset` parameter { #use-the-response-model-exclude-unset-parameter }

You can set the *path operation decorator* parameter `response_model_exclude_unset=True`:

{* ../../docs_src/response_model/tutorial004_py310.py hl[22] *}

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {
    return (ValueReference<K, V>) UNSET;
  }

  private enum NullEntry implements ReferenceEntry<Object, Object> {
    INSTANCE;

    @Override
    public @Nullable ValueReference<Object, Object> getValueReference() {

		gorootPath = runtime.GOROOT()
		if gorootPath != "" {
			return
		}

		// runtime.GOROOT is empty — perhaps go_android_exec was built with
		// -trimpath and GOROOT is unset. Try 'go env GOROOT' as a fallback,
		// assuming that the 'go' command in $PATH is the correct one.

		cmd := exec.Command("go", "env", "GOROOT")
		cmd.Stderr = os.Stderr
		out, err := cmd.Output()
		if err != nil {