void
foo (x)
    SV *    x
        PREINIT:
        I32* temp;
        PPCODE:
        temp = PL_markstack_ptr++;
        foo(x);
        if (PL_markstack_ptr != temp) {
          /* truly void, because dXSARGS not invoked */
          PL_markstack_ptr = temp;
          XSRETURN_EMPTY; /* return empty stack */
        }
        /* must have used dXSARGS; list context implied */
        return; /* assume stack size is correct */

##</code><code>##

void
foo (x)
    SV *    x
        CODE:
        foo(x);
        XSRETURN_EMPTY;