table.copy

function table.copy(tbl)
  checkArg(1, tbl, "table")
  local seen = {}
  local function copy(t, to)
    to = to or {}
    for k, v in pairs(t) do
      if type(v) == "table" then
        if not seen[v] then
          seen[v] = {}
          to[k] = seen[v]
          copy(v, seen[v])
        end
      else
        to[k] = v
      end
    end
    return to
  end
  return copy(tbl, {})
end