code: function hydrateLayerGroup(layersDef, layerGroup) {
    const hydrated_group = [];
    for (let i = 0, ii = layerGroup.length; i < ii; i++) {
      // hydrateLayer checks for "ref"
      hydrated_group.push(hydrateLayer(layersDef, layerGroup[i]));
    }
    return hydrated_group;
  }
docs: Hydrate a layer group
  Normalizes all the ref layers in a group.
  
  @param {Object[]} layersDef All layers defined in the Mapbox GL stylesheet.
  @param {Object[]} layerGroup Subset of layers to be rendered as a group.
  
  @returns {Object[]} An array with the ref layers normalized.