code: public static String assertBasic(String symbol, IContext context) {
          if (symbol.contains("_utils.sep(\"")) return symbol;// Rythm builtin expression TODO: generalize
          //String s = Token.stripJavaExtension(symbol, context);
          //s = S.stripBrace(s);
          String s = symbol;
          boolean isSimple = Patterns.VarName.matches(s);
          IContext ctx = context;
          if (!isSimple) {
              throw new TemplateParser.ComplexExpressionException(ctx);
          }
          return s;
      }
docs: Return symbol with transformer extension stripped off
  
  @param symbol
  @param context
  @return the symbol