function mutstring (s)
  assert(type(s) == "string", "string expected")
  local ms = s or ""
  local u = newproxy(true)
  local mt = getmetatable(u)
  local relatpos = function(p)
    local l = #ms
    if p < 0 then p = l + p + 1 end
    if p < 1 then p = 1 end
    return p, l
  end
  mt.__index = function(_, k)
    assert(type(k) == "number", "number expected as key")
    local k, l = relatpos(k)
    if k <= l then
      return ms:sub(k, k)
    end
  end
  mt.__newindex = function(_, k, v)
    assert(type(k) == "number", "number expected as key")
    assert(type(v) == "string" and #v == 1, "character expected as value")
    local k, l = relatpos(k)
    if k <= l + 1 then
      ms = ms:sub(1, k - 1) .. v .. ms:sub(k + 1, l)
    end
  end
  mt.__len = function(_) return #ms end
  mt.__tostring = function(_) return ms end
  return u
end

s = mutstring("Lua race")
print(s[5])

s[5] = "f"
print(s)

2.

local mt = getmetatable('')
local index = mt.__index
	mt.__index = function (str, ...)
    local k = ...
    if 'number' == type(k) then
        return index.sub(str, k, k)
    else
        return index[k]
    end
end

local a = 'abc'

for k = 1, #a do
    print(a[k])
end


print(a:upper())