(class-of v) --> #<<class> <2D-vector> 40241ac0>
<2D-vector>  --> #<<class> <2D-vector> 40241ac0>
(class-of 1) --> #<<class> <integer> 401b2a98>
<integer>    --> #<<class> <integer> 401b2a98>

(is-a? v <2D-vector>) --> #t