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A subtype is a class of objects that share the same metatable, in
particular when the objects belong to a type of which the metatable
can only be set via the debug library.

I have written a module `subtype.c` of which the complete code is this:

/* A subtype is a standard Lua type with a subtype-specific metatable.

   subtype = require"subtype"

   bits = subtype('number',bits_mt)
   j = bits(10)
0x0000000a     number  true
10    number   nil

   utf8 = subtype('string',utf8_mt)
   t = "skêr"
   s = utf8(t)
5    string   true
4    string   false

I.e. the effect is as if one said "debug.setmetatable(j,bits_mt)"
and "debug.setmetatable(s,utf8_mt)" but without affecting
other numbers and strings.

That's right, it consists of only a comment. The reason is that
I have only a foggy notion of what to do next. Clearly the real
object must be a userdata wrapping a single Lua TValue. Also
the built-in type function would need to be replaced. But it seems
to be impossible to do what I want in the C API, one would need
to use lobject.h.

Am I missing something?