title: Write data into a specific workbook sheet
body: <p>I have a data frame e.g.</p>
  
  <pre><code>a=1:10 
  b=31:40
  c=data.frame(a=a,b=b)
  </code></pre>
  
  <p>and I will need to write this data frame into a specific Excel sheet ("Sheet1").</p>
  
  <p>I am using WriteXLS now but this function always overrides the entire excel file and thus deletes other sheets.
  How can I append to the sheet without overwriting previous entries?</p>