final CommonTree ast = (CommonTree)stylesheet.getTree();
final CommonTreeNodeStream nodes = new CommonTreeNodeStream(ast);
nodes.setTokenStream(tokens);
// walk the tree and build definitions
final CSSTree treeWalker = new CSSTree(nodes);
treeWalker.stylesheet();
problems.addAll(lexer.problems);
problems.addAll(parser.problems);
problems.addAll(treeWalker.problems);