function geojson([x, y, z], layer, filter = () => true) {
if (!layer) return;
const features = [];
for (let i = 0; i < layer.length; ++i) {
const f = layer.feature(i).toGeoJSON(x, y, z);
if (filter.call(null, f, i, features)) features.push(f);
}
return {type: "FeatureCollection", features};
}