docs: function layersPBFReader(tag, layers, pbf) {
    if (tag === 3) {
      const layer = {
        keys: [],
        values: [],
        features: []
      };
      const end = pbf.readVarint() + pbf.pos;
      pbf.readFields(layerPBFReader, layer, end);
      layer.length = layer.features.length;
      if (layer.length) {
        layers[layer.name] = layer;
      }
    }
  }