code: function segment(geojson) {
      var segments = [];
      geojson.features.forEach(function(f) {
          if (f.geometry && f.geometry.type === 'LineString') {
              var coords = f.geometry.coordinates;
              if (f.properties.coordTimes ||
                  f.properties.heartRates) {
                  var heartRates = f.properties.heartRates,
                      coordTimes = f.properties.coordTimes;
                  for (var i = 0; i < coords.length - 1; i++) {
                      segments.push({
                          type: 'Feature',
                          geometry: {
                              type: 'LineString',
                              coordinates: [coords[i], coords[i + 1]]
                          },
                          properties: {
                              heartRates: coordTimes && [heartRates[i], heartRates[i + 1]],
                              coordTimes: coordTimes && [coordTimes[i], coordTimes[i + 1]],
                          }
                      });
                  }
              }
          }
      });
      return {
          type: 'FeatureCollection',
          features: segments
      };
  }
docs: @alias segment
  
  Given a [GeoJSON](http://geojson.org/) produced by [toGeoJSON](https://github.com/mapbox/togeojson)
  from GPX, split LineString geometries into two-vertex linestrings with properties
  of heartRate and time before & after.
  @param {Object} geojson a GeoJSON FeatureCollection object output by toGeoJSON
  @returns {Object} a GeoJSON FeatureCollection of two-vertex linestrings