int callComp( SV* cmp, int a, int b ) {
    int count, rv;
    dSP; ENTER; SAVETMPS; 
    PUSHMARK(SP);
    XPUSHs( sv_2mortal( newSViv( a ) ) );
    XPUSHs( sv_2mortal( newSViv( b ) ) );
    PUTBACK; 
    if( ( count = call_sv( cmp, G_SCALAR ) ) != 1 ) 
        croak( "Bad comparator" );
    SPAGAIN; 
    rv = POPi;
    FREETMPS; LEAVE;
}