code: func (l *Line) ToGeoJSON() *geojson.Feature {
  	return geojson.NewLineStringFeature([][]float64{
  		{l.a[0], l.a[1]},
  		{l.b[0], l.b[1]},
  	})
  }
docs: // ToGeoJSON creates a new geojson feature with a linestring geometry
  // containing the two points.