The border function is already beautifully documented, but not named,
in the manual. However, it is hidden by the __len metamethod, and in
particular cannot be called by that metamethod. It would be nice to
have it exposed, and `table.border` seems like a natural place for it.
To achieve that while conforming to the custom that the libraries use
only lualib.h and lauxlib.h, one would need `int lua_border(lua_State
*L, int index)`.