code: function addLayer(state, action) {
    // TODO: Maybe decide on what a "default case" is in
    //       order to support easier dev.
    const new_layer = Object.assign({
      paint: {},
      metadata: {},
    }, action.layerDef);
  
    // add a layer name if specified in the action.
    if (action.layerTitle) {
      new_layer.metadata[TITLE_KEY] = action.layerTitle;
    }
  
    return placeLayer(state, new_layer, action.positionId);
  }
docs: Add a layer to the state.
  @param {Object} state Current state.
  @param {Object} action Action to handle.
  
  @returns {Object} The new state.