title: What are :+ and &:+ in Ruby?
body: <p>I've seen these several times but I can't figure out how to use them. The pickaxe says that these are special shortcuts but I wasn't able to find the syntactical description.</p>
  
  <p>I've seen them in such contexts:</p>
  
  <pre><code>[1,2,3].inject(:+)
  </code></pre>
  
  <p>to calculate sum for example.</p>