Hello,

From the Guile manual ``6.1 Overview of the Guile API'',

>    For some Scheme functions, some last arguments are optional; the
> corresponding C function must always be invoked with all optional
> arguments specified.  To get the effect as if an argument has not been
> specified, pass ‘SCM_UNDEFINED’ as its value.  You can not do this for
> an argument in the middle; when one argument is ‘SCM_UNDEFINED’ all the
> ones following it must be ‘SCM_UNDEFINED’ as well.

Therefore, we can check if opt_arg2 has the value SCM_UNDEFINED, to
decide if we have received an optional argument. The code is in the
attachment: