title: Pass a variable to sleep in vimscript
body: <p>I've been using vim for too many years to count, but I never have really learned vimscript very well. I'm trying now.</p>
  
  <p>Anyway, I would like to pass a variable amount of time to the sleep function. I also want to manipulate that value before I pass it along. Here's a simple example.</p>
  
  <pre><code>function! wait(mil)
      let timetowait = mil . "m"
      sleep timetowait
  endfunction
  </code></pre>
  
  <p>Even if I try prefixing timetowait with l: it says, "Invalid argument: l:timetowait".</p>
  
  <p>What's the right way of passing the value of a variable to sleep?</p>