code: 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();
  			}
  		}
  		
  		node.children.forEach(squash);
  		
  		return node;
  	}
docs: Optimizes tree node: replaces empty nodes with their children
  @param {AbbreviationNode} node
  @return {AbbreviationNode}