sub kirst {
        my ($fja, $a, $b) = @_;
        die "Nera saknu?" if ($fja->($a) * $fja->($b) > 0);
        my $c = $a - (($b - $a) * $fja->($a)) / ($fja->($b) - $fja->($a));
        if (abs($fja->($c)) < 0.00001) { # 0.00001 yra epsilon
                return $c;
        } elsif ($fja->($a) * $fja->($c) < 0) {
                return kirst($fja, $a, $c);
        } else {
                return kirst($fja, $c, $b);
        }
}