kernel
The geometric primitives, like the point type, are defined in a kernel.
traits
predicates & constructions
Besides the types we see predicates like the orientation test for three points, and constructions like the distance
and midpoint computation. A predicate has a discrete set of possible results, whereas a construction produces
either a number, or another geometric entity.
concepts & models
A concept is a set of requirements on a type, namely that it has certain nested types, certain member functions,
or comes with certain free functions that take the type as it. A model of a concept is a class that fulfills the
requirements of the concept.
Concept是由一组需求定义的类型的抽象。概念是由一组需求定义的类型的抽象。如果任何具体类型满足与概念相对应的一组需求，则称为这个概念对应的Model。