Making a string indexing metamethod

["Reuben Thomas" <rrt@...>]

To allow string methods to continue to work, you can't just override  
__index. After some thought, I came up with this:

-- @func __index: Give strings a subscription operator
--   @param s: string
--   @param n: index
-- @returns
--   @param s_: string.sub (s, n, n)
local oldmeta = getmetatable("").__index
getmetatable("").__index =
  function (s, n)
    if type (n) == "number" then
      return sub (s, n, n)
    else
      return oldmeta[n]
    end
  end

What are the downsides?

--
http://rrt.sc3d.org/