title: Filter groups in dplyr that exclusively contain specific combinations of values
body: <p>Given a table like:</p>
  
  <pre><code>  id value
  1  1     a
  2  2     a
  3  2     b
  4  2     c
  5  3     c
  </code></pre>
  
  <p>I would like to filter for:</p>
  
  <p>a) the ids that only have value a, i.e. id 1.</p>
  
  <p>b) the ids that contain a and b jointly, i.e. id 2.</p>
  
  <p>Data:</p>
  
  <pre><code>data.frame(id = c(1,2,2,2,3), value = c("a", "a", "b", "c", "c"))
  </code></pre>