int callComp( SV* cmp, SV *a, SV *b) {
    int count, rv;
    GV *first = gv_fetchpv("a", TRUE, SVt_PV);
    GV *second = gv_fetchpv("b", TRUE, SVt_PV);

	dSP; ENTER; SAVETMPS; PUSHMARK(SP);

	XPUSHs( a );
	XPUSHs( b );

    PUTBACK; 

	GvSV(first)  = a;
	GvSV(second) = b;

    if( ( count = call_sv( cmp, G_SCALAR|G_NOARGS) ) != 1 )
        croak( "Bad comparator" );
        
    SPAGAIN; 
    rv = POPi;
    FREETMPS; LEAVE;
}