local function sbox (untrusted_code)

    -- make environment

    local env = {}

    -- run code under environment

    local untrusted_function, message = loadstring(untrusted_code)

    if not untrusted_function then return nil, message end

    setfenv(untrusted_function, env)

    local success, result = pcall(untrusted_function)

    if success then

        return result

    else

        return nil, result

    end

end

-- ---- ----

local function dc (args)

    if not args then return nil, "Give me an expression" end

    -- Downloaded from http://www.math.bas.bg/bantchev/place/rpn/rpn.lua.html

    tb = {}  z = 0

    for tk in string.gmatch(args,'%S+') do

      if string.find(tk,'^[-+*/]$')  then

        if 2>#tb then z = nil break end

        y,x = table.remove(tb),table.remove(tb)

        loadstring('z=x'..tk..'y')()

      else

        z = tonumber(tk)  if z==nil then break end

      end

      table.insert(tb,z)

    end

    n = #tb

    if n==1 and z then return z

    elseif n>1 or z==nil then return nil, "dc: Error!" end

end

local function calc (args)

    if not args then return nil, "Give me an expression" end

    local f, msg = sbox("return "..args)

    if not f then

        return nil, "calc: Cannot evaluate expression"

    end

    return tostring(f)

end

mcbot_register_command("dc", dc)

mcbot_register_command("calc", calc)