I am on the current master branch. After reading section 6.6.18.4 (Meta-Vtables) of the reference manual there remains an open question.
A structure X made from a meta-vtable becomes a meta-vtable if standard-vtable-fields is a prefix of the layout in X's field 0. And if X is a meta-vtable, every structure with X as a vtable must have a valid layout in field 0. Suppose the following: standard-vtable-fields accidentally is a prefix of some valid layout string l. Then (define v (make-vtable l)) (make-struct/no-tail v 'a) raises an exception, because 'a is not a valid layout. But v may not be intended to be a meta-vtable, just a vtable. The reference manual does mention why that can not happen, or why is it accepted that it may happen.
