title: Combine two or more columns in a dataframe into a new column with a new name
body: <p>For example if I have this:</p>
  
  <pre><code>n = c(2, 3, 5) 
  s = c("aa", "bb", "cc") 
  b = c(TRUE, FALSE, TRUE) 
  df = data.frame(n, s, b)
  
    n  s     b
  1 2 aa  TRUE
  2 3 bb FALSE
  3 5 cc  TRUE
  </code></pre>
  
  <p>Then how do I combine the two columns <code>n</code> and <code>s</code> into a new column named <code>x</code> such that it looks like this:</p>
  
  <pre><code>  n  s     b     x
  1 2 aa  TRUE  2 aa
  2 3 bb FALSE  3 bb
  3 5 cc  TRUE  5 cc
  </code></pre>