Geometry is the base class. It's an abstract class.
The instantiable subclasses of Geometry are restricted to zero-, one-,
and two-dimensional geometric objects that exist in
two-dimensional coordinate space. All instantiable geometry classes are
defined so that valid instances of a geometry class are topologically closed
(that is, all defined geometries include their boundary).

The base Geometry class has subclasses for Point,
Curve, Surface, and GeometryCollection:


 --- Point represents zero-dimensional objects.

 --- Curve represents one-dimensional objects, and has subclass
LineString, with sub-subclasses Line and LinearRing.

 --- Surface is designed for two-dimensional objects and
has subclass Polygon.

 --- GeometryCollection
has specialized zero-, one-, and two-dimensional collection classes named
MultiPoint, MultiLineString, and MultiPolygon
for modeling geometries corresponding to collections of
Points, LineStrings, and Polygons, respectively.
MultiCurve and MultiSurface are introduced as abstract superclasses
that generalize the collection interfaces to handle Curves and Surfaces.


Geometry, Curve, Surface, MultiCurve,
and MultiSurface are defined as non-instantiable classes.
They define a common set of methods for their subclasses and
are included for extensibility.

Point, LineString, Polygon, GeometryCollection,
MultiPoint, MultiLineString, and
MultiPolygon are instantiable classes.
