assert problems != null : "Problem collection can't be null";
try
{
// parse and build tree
final CSSLexer lexer = new CSSLexer(input);
final CommonTokenStream tokens = new CommonTokenStream(lexer);
final CSSParser parser = new CSSParser(tokens);
final CSSParser.stylesheet_return stylesheet = parser.stylesheet();
final CommonTree ast = (CommonTree)stylesheet.getTree();
final CommonTreeNodeStream nodes = new CommonTreeNodeStream(ast);