title: Julia adding indexed arrays
body: <p>I have two arrays in Julia: <code>A</code>, which has size <code>(2,2,3)</code> and <code>a</code> which has size <code>(2,2)</code>.</p>
  
  <p>If I try to perform the following operation</p>
  
  <pre><code>a += A[:,:,1]
  </code></pre>
  
  <p>I get the error</p>
  
  <pre><code>ERROR: LoadError: DimensionMismatch("dimensions must match")
  in promote_shape at operators.jl:231
  in promote_shape at operators.jl:227
  in + at arraymath.jl:96
  </code></pre>
  
  <p>I looked at the array documentation and couldn't figure out how to get around this issue.</p>