text: Q:
  
  Julia parallel for loop with two reductions
  
  I would like to perform two reductions in a parallel for loop in Julia. I am trying to compute the error in a random forest inside the parallel for loop as each tree is built. Any ideas?
  Current:
  forest = @parallel (vcat) for i in 1:ntrees
      inds = rand(1:Nlabels, Nsamples)
      build_tree(labels[inds], features[inds,:], nsubfeatures)
  end
  
  What I want, intuitively is to do an addition inside this for loop as well to get the out of bag error. This is how I would wish for it to work:
  forest, ooberror = @parallel (vcat, +) for i in 1:ntrees
      inds = rand(1:Nlabels, Nsamples)
      tree = build_tree(labels[inds], features[inds,:], nsubfeatures)
      error = geterror(ids, features, tree)
      (tree, error)
  end
  
  A:
  
  Using a type might be best in terms of simplicity and clarity, e.g.
  type Forest
    trees :: Vector
    error
  end
  join(a::Forest, b::Forest) = Forest(vcat(a.trees,b.trees), a.error+b.error)
  
  #...
  
  forest = @parallel (join) for i in 1:ntrees
      inds  = rand(1:Nlabels, Nsamples)
      tree  = build_tree(labels[inds], features[inds,:], nsubfeatures)
      error = geterror(ids, features, tree)
      Forest(tree, error)
  end
  
  