old_contents: <!-- create the map, set the zoom, add it to the 'map' div element -->
      var map = L.map('map')
          .setView([36, -93], 3);
          L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1IjoibGFuZ2Rvbm1zIiwiYSI6ImNpZmQxdzE1dzRyd2RzbWx4bnp2Ym5maDcifQ.mrTKjuh7OcKwTDWc95OlZQ', {
              maxZoom: 18,
              attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, ' +
                  '<a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, ' +
                  'Imagery © <a href="http://mapbox.com">Mapbox</a>',
              id: 'mapbox.streets'
          }).addTo(map);
          
  <!-- create the geocoder -->
      new L.Control.GeoSearch({
          provider: new L.GeoSearch.Provider.OpenStreetMap(),
          position: 'topleft',
          showMarker: true,
          retainZoomLevel: false,
      }).addTo(map);
  
  <!-- data location-->
      var csvrider = "/carpool-map/data/unmatched-rider.csv";
      var csvdriver = "/carpool-map/data/unmatched-driver.csv";
  
  <!-- create a geojson layer from the csv file -->
      var omniParseRider = omnivore.csv(csvrider);
      var omniParseDriver = omnivore.csv(csvdriver);
      
  <!-- load these layers into a group for the layer control -->
      var overlayGroup = L.layerGroup([omniParseRider, omniParseDriver]);
      
  <!-- create "label": key for layers -->    
      var overlaylabels = {
          "Riders": omniParseRider,
          "Drivers": omniParseDriver,
      };
  
  <!-- load the label, then the layer from the layer group -->
      L.control.layers(overlaylabels).addTo(map);
new_contents: <!-- create the map, set the zoom, add it to the 'map' div element -->
      var map = L.map('map')
          .setView([36, -93], 3);
          L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1IjoibGFuZ2Rvbm1zIiwiYSI6ImNpZmQxdzE1dzRyd2RzbWx4bnp2Ym5maDcifQ.mrTKjuh7OcKwTDWc95OlZQ', {
              maxZoom: 18,
              attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, ' +
                  '<a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, ' +
                  'Imagery © <a href="http://mapbox.com">Mapbox</a>',
              id: 'mapbox.streets'
          }).addTo(map);
          
  <!-- create the geocoder -->
      new L.Control.GeoSearch({
          provider: new L.GeoSearch.Provider.OpenStreetMap(),
          position: 'topleft',
          showMarker: true,
          retainZoomLevel: false,
      }).addTo(map);
  
  <!-- data location-->
      var csvrider = "/carpool-map/data/unmatched-rider.csv";
      var csvdriver = "/carpool-map/data/unmatched-driver.csv";
  
  <!-- create a geojson layer from the csv file, .addTo(map) turns these layers on by default-->
      var omniParseRider = omnivore.csv(csvrider).addTo(map);
      var omniParseDriver = omnivore.csv(csvdriver).addTo(map);
      
  <!-- load these layers into a group for the layer control -->
      var overlayGroup = L.layerGroup([omniParseRider, omniParseDriver]);
      
  <!-- create "label": key for layers -->    
      var overlaylabels = {
          "Riders": omniParseRider,
          "Drivers": omniParseDriver,
      };
      
  <!-- load the label, then the layer from the layer group -->
  <!-- loads the map controller, using the .addTo(map) method when creating the layer initially determines if on/off at start-->
      L.control.layers(null, overlaylabels).addTo(map);
subject: Configure Layer Control for Map