function App.open_for_writing(filename)
  App.filesystem[filename] = ''
  return {
    write = function(self, s)
              App.filesystem[filename] = App.filesystem[filename]..s
            end,
    close = function(self)
            end,
  }
end

function App.open_for_writing(filename)
  App.filesystem[filename] = ''
  return {
    write = function(self, s)
              App.filesystem[filename] = App.filesystem[filename]..s
            end,
    close = function(self)
            end,
  }
end