default boolean isChild(final Tree<?, ?> node) {
		requireNonNull(node);
		return childCount() != 0 &&
			node.parent().equals(Optional.of(self()));
	}
Return {@code true} if the given {@code node} is a child of {@code this}
node.

@param node the other node to check
@return {@code true} if {@code node}is a child, {@code false} otherwise
@throws NullPointerException if the given {@code node} is {@code null}
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java