mt_table = {
  __add = function(a, b)
    local tbl = {}
    for k,v in pairs(a) do
      tbl[k] = v
    end
    for k,v in pairs(b) do
      if not tbl[k] then
        tbl[k] = v
      else
        tbl[#tbl+1] = v
      end
    end
    return tbl
  end,
  __sub = function(a, b)
    return #a - #b
  end
}
tbl1 = {5,9,6,7,2,1,5,4}
tbl2 = {5,8,5,2,1,5,3,4}
setmetatable(tbl1, mt_table)
setmetatable(tbl2, mt_table)
tbl3 = tbl1 + tbl2
print(table.concat(tbl3, " , "))
num = tbl3 - tbl2
print(num)