Primitive
Marker for a shape that is itself a CGA element.
Implemented by the five 2D blades: FlatPoint, Line, RoundPoint, Dipole, Circle. Sealed, so a when over a Primitive is exhaustive without an else — a new blade is a compile error at every dispatch site. Composites, the other kind of Shape, are built from primitives by composition and carry no blade components of their own.