find

auto index = [1, 2, 3, 4].sliced;
auto data = [2.1, 3.4, 5.6, 7.8].sliced;
auto series = index.series(data);

if (auto bi = series.find(3))
{
    assert(series.data[$ - bi] == 5.6);
}
else
{
    assert(0);
}

assert(series.find(0) == 0);