title: How to DROP columns from ffdf object ? (R)
body: <p>Could I easily drop column of <code>ffdf</code> object ?</p>
  
  <pre><code>library(ff);library(ffbase)    
  irisdf=as.ffdf(iris)
  </code></pre>
  
  <p>How to contain only <code>Sepal.length</code> and <code>Species</code> columns ?</p>