luajit

isSymbol = (function ()
	local symbols = [=[~`!@#$%^&*()_-+=|\{[}]:;"'<,>?/]=]
	return function(char)
		for ix = 1, string.len(symbols), 1 do
			if char == string.sub(symbols, ix, ix) then
				return true
			end
		end
		return false
	end
end)()
process = function(input)
	if type(input) == "string" then
  	input = io.open(input)
  end
  local charset = {}
  for line in input:lines() do
  	for ix = 1, string.len(line), 1 do
			charset[string.sub(line, ix, ix)] = true
		end
	end
	local s = ""
	for k, v in pairs(charset) do
    s = s .. k
	end
	print(s)
end
main = function()
	local ix
	ix = 1
	while arg[ix] do
		process(arg[ix])
		ix = ix + 1
	end
end
main()