title: Extract every nth element of a vector
body: <p>I would like to create a vector in which each element is the <code>i+6th</code> element of another vector.</p>
  
  <p>For example, in a vector of length 120 I want to create another vector of length 20 in which each element is value <code>i, i+6, i+12, i+18...</code> of the initial vector, i.e. I want to extract every 6th element of the original.</p>