protected void checkTreeState() {
		if (_genes == null) {
			throw new IllegalStateException(
				"Gene is not attached to a chromosome."
			);
		}
	}
Return the whole flattened tree values in breadth-first order. This method
will always return the same {@code ISeq} instance.

@return the whole flattened tree values
jenetics.ext/src/main/java/io/jenetics/ext/AbstractTreeGene.java