Class Rectangle is defined in rectangs.web, and
is derived from Polygon, using public derivation.
As noted above in Polygon Reference; Affine Transformations, class Rectangle,
like class Reg_Polygon,
currently inherits its transformation functions and
operator*=(const Transform&) from Polygon. Consequently,
the data members of a Rectangle, except for center, are
not recalculated when it's transformed. I plan to change this soon! It
will also be necessary to add the function
Rectangle::is_rectangular(), in order to test whether a
Rectangle is still rectangular.