CommonTokenStream tokens = new CommonTokenStream(lexer);
parser = new ForteParser(tokens);
reporter = new ErrorReporter();
parser.setErrorReporter(reporter);
// parser.setTokenStream(tokens);
expression_return result = parser.expression();
CommonTree tree = (CommonTree) result.getTree();
errors = parser.getNumberOfSyntaxErrors();
if (errors == 0){
if (PRINT_TREE)
System.out.println("Tree==>"+tree.toStringTree());