from.lua

local getlocal, setlocal = debug.getlocal, debug.setlocal

local function from_err(k, msg)
  local info = debug.getinfo(3)
  io.stderr:write(string.format(
    "%s:%s:%d: local variable '%s' %s in import block\n",
    info.source:sub(2),info.name,info.currentline,k,msg))
  os.exit()
end

local function from(t)
  local i = 1
  while true do
    local k, v = getlocal(2, i)
    if not k or v == nil then break end
    i = i + 1
  end
  while true do
    local k, v = getlocal(2, i)
    if k == nil then break
    elseif v  ~= nil then from_err(k, "initialized")
    elseif t[k] == nil then from_err(k, "not found.")
    end
    setlocal(2, i, t[k])
    i = i + 1
  end
end

local function test(val)
  local exp, log from (math)
  local sub from (string)
  local sin, cos, tan from (math)

  assert(exp == math.exp)
  assert(log == math.log)
  assert(sin == math.sin)
  assert(cos == math.cos)
  assert(tan == math.tan)
  assert(sub == string.sub)

  return exp(val), tan(val)
end

--local asin, acos, atan from (math)

print(test(1))

within.lua

local function getnlocal(stack)
  local i = 1
  repeat
    local k, v = debug.getlocal(stack + 1, i)
    if k then
      i = i + 1
    end
  until nil == k
  return i
end

local function within(t, n)
  local line = debug.getinfo(2).currentline
  local count = getnlocal(2)
  for i=count-n, count-1 do
    local name = debug.getlocal(2, i)
    debug.setlocal(2, i, t[name])
  end
end

local exp, log, sin, cos, tan within (math, 5)

assert(exp == math.exp)
assert(log == math.log)
assert(sin == math.sin)
assert(cos == math.cos)
assert(tan == math.tan)