Model is described by the boundary representation and consists of
the following model entities -- vertices, curves, surfaces and regions.
Region is formed by a set of not self-intersecting boundary surfaces.
Each surface is bounded by a set of curves and
stores the regions on the side of its outer and inner normal. Curve is
determined by two end vertices and keeps list of surfaces sharing this
curve. Vertex stores only the list of associated curves. This basic
topology is further restricted by geometry of model entities. Both
curves and surfaces are based on free-form representation in terms of
tensor product polynomial entities. This limits the number of curves
bounding a surface to four. Currently, the rational Bezier entities are employed for
free-form curves and surfaces representation. This allows to represent
exactly conics and quadrics. To enhance the modeling capability the
entity-to-entity fixation concept is introduced. Generally, each model
entity may be fixed to another model entity of the same or higher
dimension. The fixed
entity must coincide with appropriate parametric subentity of parent
model entity except the boundary of that parent entity. Each
model entity is keeping the list of entities fixed to it. No further
topological information are required for description of valid domain
of arbitrary complexity.

*Daniel Rypl
2005-12-03*