title: R: return row and column numbers of matches in a data frame
body: <p><code>emperor &lt;- rbind(cbind('Augustus','Tiberius'),cbind('Caligula','Claudius'))</code></p>
  
  <p>How do I return the row and column numbers of all the cells that contain the sequence 'us', i.e. [1,1], [1,2], [2,2]?</p>