default int indexOf(final Tree<?, ?> child) {
		int index = -1;
		for (int i = 0, n = childCount(); i < n && index == -1; ++i) {
			if (childAt(i).identical(child)) {
				index = i;
			}
		}

		return index;
	}
Returns the index of the specified child in this node's child array, or
{@code -1} if {@code this} node doesn't contain the given {@code child}.
This method performs a linear search and is O(n) where {@code n} is the
number of children.

@param child  the TreeNode to search for among this node's children
@throws NullPointerException if the given {@code child} is {@code null}
@return the index of the node in this node's child array, or {@code -1}
        if the node could not be found
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java