title: under lua, how can I if check file attributes only if the LuaFileSystem package is installed
body: <p>I have some code which uses LuaFileSystem. However not all systems it will be run on have LuaFileSystem installed. I would like to check if it is installed, and only run the code if it is. Something like this (but this fails and states lfs is a null value)</p>
  
  <pre><code>local lfsExists, lfs = pcall(function () require "lfs" end)
  if lfsExists then
      local lastUpdateTime = lfs.attributes( mapFilePName ).modification
  end
  </code></pre>