Openings

An Opening is defined by one or more boundary curves that form a closed boundary. An Opening is one of Internal Objects of a Surface. Boundaries of an Opening must lie in its mother Surface (i.e., it is not possible to define an Opening over multiple Surfaces.

An Opening can be filled with another Surface, i.e., boundaries of an Opening can serve simultaneously as boundaries of this Surface. This allows us to define a problem where inside of a Surface1 is another Surface2 (for example, different material). One needs to first define an Opening in Surface1 and then to fill it with Surface2.