title: Vim change filetype of file with extension `.pl` to prolog instead of perl
body: <p>The default filetype of a file with <code>.pl</code> extension is perl.
  However I use prolog more often so every time I open a new file I have to set it manually by running vim command</p>
  
  <p><code>:set filetype=prolog</code></p>
  
  <p>I tried to automate it by inserting the the following line into <code>~/.vimrc</code> </p>
  
  <p><code>autocmd BufNewFile,BufRead *.pl   set syntax=prolog</code></p>
  
  <p>also tried inserting this line in <code>.vim/ftdetect/pl.vim</code> file in vain.</p>
  
  <p>What should be done to set <code>.pl</code> files to prolog type instead of perl?</p>