Can it also have an interface to query how many values a given box is holding?

Without that, any generic code to deal with boxes must receive the unboxed values

as a list.  Although clever compilers may eliminate construction of intermediate lists

in some cases, it'll be easier if one can get the information (e.g. one can have fast-path

for single-valued box, or check the box argument and reject early if the passed box

has different values than expected.)