public Object exec(List args) throws TemplateModelException 
{
                Object obj;
                String sub;
                int fidx;
                int ln  = args.size();
                if (ln == 0) {
                    throw new TemplateModelException(
                            "?index_of(...) expects at least one argument.");
                }
                if (ln  > 2) {
                    throw new TemplateModelException(
                            "?index_of(...) expects at most two arguments.");
                }
                obj = args.get(0);       
                if (!(obj instanceof TemplateScalarModel)) {
                    throw new TemplateModelException(
                            "?index_of(...) expects a string as "
                            + "its first argument.");
                }
                sub = ((TemplateScalarModel) obj).getAsString();
                if (ln  > 1) {
                    obj = args.get(1);
                    if (!(obj instanceof TemplateNumberModel)) {
                        throw new TemplateModelException(
                                "?index_of(...) expects a number as "
                                + "its second argument.");
                    }
                    fidx = ((TemplateNumberModel) obj).getAsNumber().intValue();
                } else {
                    fidx = 0;
                }
                return new SimpleNumber(s.indexOf(sub, fidx));
}