source-layer': 'units',
          minzoom: 6,
          maxzoom: 10,
          paint: {
            'line-color': '#555555',
            'line-width': {
              'stops': [
                [6, 0.1],
                [7, 0.2],
                [8, 0.5],
                [9, 0.6]
              ]
            }
          }
        },
        {
          id: 'medium_lines',
          type: 'line',
          source: 'burwell_medium',
          'source-layer': 'lines',
          minzoom: 6,
          maxzoom: 9,