[luatex] \insert numbers

[Joseph Wright]

Hello all,

In trying to answer

http://tex.stackexchange.com/questions/40361/how-can-the-extended-register-pool-be-used-for-newinsert

I had a read through the LuaTeX manual and did a few quick tests. I
can't find it documented, but LuaTeX seems to accept a 16-bit number for
\insert classes. Is this correct, is it by design and if so have I
missed it in the manual?
-- 
Joseph Wright