Categories are the main GAP objects in CAP. They are used to
associate GAP objects which represent objects and morphisms with
their category. By associating a GAP object to the category, one of
two filters belonging to the category (ObjectFilter/MorphismFilter)
are set to true. Via Add methods, functions for specific existential
quantifiers can be associated to the category and after that can be
applied to GAP objects in the category. A GAP category object also
knows which constructions are currently possible in this category.