title: Compact way of writing (a + b == c or a + c == b or b + c == a)
body: <p>Is there a more compact or pythonic way to write the boolean expression</p>
  
  <pre><code>a + b == c or a + c == b or b + c == a
  </code></pre>
  
  <p>I came up with </p>
  
  <pre><code>a + b + c in (2*a, 2*b, 2*c)
  </code></pre>
  
  <p>but that is a little strange.</p>