function registered with scm_c_define_gsubr: how can i handle an optional parameter?

[Pierre LINDENBAUM]

Hi all,

(cross posted on SO: https://stackoverflow.com/questions/47797521 )

I've defined a guile C function:

    static SCM myfun(SCM arg1,SCM opt_arg2)
      {
      return SCM_BOOL_T;
      }

registered with

    scm_c_define_gsubr ("myfun", 1, 1, 0, myfun);

there is one optional argument. How can I detect if opt_arg2 has been 
used ?

in

    (myfun 1)

or

    (myfun 1 2)


Thanks !