default int leafCount() {
		return (int)leaves().count();
	}
Returns the total number of leaves that are descendants of this node.
If this node is a leaf, returns {@code 1}. This method is {@code O(n)},
where n is the number of descendants of {@code this} node.

@see #isLeaf()
@return the number of leaves beneath this node
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java