public boolean eval(FormInstance model, EvaluationContext evalContext) {
   return XPathFuncExpr.toBoolean(evalRaw(model, evalContext)).booleanValue();
 }
 public String evalReadable(FormInstance model, EvaluationContext evalContext) {
   return XPathFuncExpr.toString(evalRaw(model, evalContext));
 }
 public Object evalRaw(FormInstance model, EvaluationContext evalContext) {
   return XPathFuncExpr.unpack(expr.eval(model, evalContext));
 }