title: How to stack multiple columns using tidyverse
body: <p>I have a data frame like this in wide format</p>
  
  <pre><code>setseed(1)
  df = data.frame(item=letters[1:6], field1a=sample(6,6),field1b=sample(60,6),
                  field1c=sample(200,6),field2a=sample(6,6),field2b=sample(60,6),
                  field2c=sample(200,6))
  </code></pre>
  
  <p>what would be the best way to stack all a columns together and all b together and all c together like this</p>
  
  <pre><code>items fielda fieldb fieldc
      a     2      52    121
      a     1      44     57
  </code></pre>