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Name	Kind	Description
_Curve	class instance	The "_Curve" element is the abstract head of the substituition group for all (continuous) curve elements.
_CurveSegment	class instance	The "_CurveSegment" element is the abstract head of the substituition group for all curve segment elements, i.e. continuous segments of the same interpolation mechanism.
_GeometricPrimitive	class instance	The "_GeometricPrimitive" element is the abstract head of the substituition group for all (pre- and user-defined) geometric primitives.
_Geometry	class instance	The "_Geometry" element is the abstract head of the substituition group for all geometry elements of GML 3. This includes pre-defined and user-defined geometry elements. Any geometry element must be a direct or indirect extension/restriction of AbstractGeometryType and must be directly or indirectly in the substitution group of "_Geometry".
_GML	class instance	Global element which acts as the head of a substitution group that may include any element which is a GML feature, object, geometry or complex value
_Object	class instance	This abstract element is the head of a substitutionGroup hierararchy which may contain either simpleContent or complexContent elements. It is used to assert the model position of "class" elements declared in other GML schemas.
_Ring	class instance	The "_Ring" element is the abstract head of the substituition group for all closed boundaries of a surface patch.
_Surface	class instance	The "_Surface" element is the abstract head of the substituition group for all (continuous) surface elements.