Optimizes tree node: replaces empty nodes with their children
@param {AbbreviationNode} node
@return {AbbreviationNode}
function squash(node) {
		for (var i = node.children.length - 1; i >= 0; i--) {
			/** @type AbbreviationNode */
			var n = node.children[i];
			if (n.isGroup()) {
				n.replace(squash(n).children);
			} else if (n.isEmpty()) {
				n.remove();
			}
		}
		
		_.each(node.children, squash);
		
		return node;
	}