An instance of this class represents a pair of geodetic coordinates (lat/lon).

class LatLon {

 double _lat;
 double _lon;

 /**
  * Creates a point with coordinates [lat] and [lon].
  *
  * Throws [ArgumentError] if either [lat] or [lon] isn't in the expected
  * range.
  */
 LatLon(num lat, num lon) {
   if (!isValidLat(lat)) throw new ArgumentError("invalid lat value, got $lat");
   if (!isValidLon(lon)) throw new ArgumentError("invalid lon value, got $lon");
   this._lat = lat.toDouble();
   this._lon = lon.toDouble();
 }

 /**
  * A (lon/lat) point at position (0,0).
  */
 LatLon.origin() :
   _lat = 0.0, _lon = 0.0;

 /**
  * Creates a lat/lon point from another value [other].
  * [other] must not be null.
  *
  * ## [other] - possible values
  *
  * * another [LatLon]
  * * a [List] with two numbers lat, lon
  * * a [Map] with two properties `lat` and `lon` of type [num].
  *
  * ## Examples
  *
  *     var ll1 = new LatLon.from([0, 12.32]);
  *     var ll2 = new LatLon.from({"lat": 0, "lon": 12.32});
  *     var ll3 = new LatLon.from(ll1);
  */
 factory LatLon.from(other) {
   if (other == null) throw new ArgumentError("other must not be null");
   if (other is List) {
     return new LatLon._fromList(other);
   } else if (other is Map) {
     return new LatLon._fromMap(other);
   } else if (other is LatLon) {
     return new LatLon(other.lat, other.lon);
   } else {
     throw new ArgumentError("can't create a LonLat from $other");
   }
 }

 factory LatLon._fromList(l) {
   if (l.length != 2)  {
     throw new ArgumentError(
       "expected list with exactly two arguments, got ${l.length}"
     );
   }
   //TODO: check arguments
   return new LatLon(l[0], l[1]);
 }

 factory LatLon._fromMap(map) {
   //TODO: check arguments
   return new LatLon(map["lat"], map["lon"]);
 }

 /// the latitue
 double get lat => _lat;

 /// the longitude
 double get lon => _lon;

 bool operator ==(other) {
   if (other == null || other is! LatLon) return false;
   return lat == other.lat && lon == other.lon;
 }

 String toString() => "{LatLon: lon=$lon, lat=$lat}";
}