I would like to expose a C++ class with Rcpp modules that has two constructors with the same number of parameters. As documented in the [Rcpp modules](http://dirk.eddelbuettel.com/code/rcpp/Rcpp-modules.pdf) vignette, this can be achieved by using a validator function as the second argument to `.constructor` with the type

```
typedef bool (*ValidConstructor)(SEXP*,int);

```

Could somebody provide an example of how it is supposed to be used? In the vignette, there is only a TODO note that an example should be included there.