title: repeating vector of letters
body: <p>Is there a function to create a repeating list of letters in R?</p>
  
  <p>something like </p>
  
  <pre><code>letters[1:30]
  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
  [20] "t" "u" "v" "w" "x" "y" "z" NA  NA  NA  NA
  </code></pre>
  
  <p>but instead of NA, I would like the output to continue  aa, bb, cc, dd ... </p>