title: Output something other than '0 pruned nodes'
body: <p>Every time I've used <code>xgboost</code> (not only with python), the training messages always include "0 pruned nodes" on each line.  For example:</p>
  
  <pre><code>import pandas as pd
  from sklearn import datasets
  import xgboost as xgb
  iris = datasets.load_iris()
  dtrain = xgb.DMatrix(iris.data, label = iris.target)
  params = {'max_depth': 10, 'min_child_weight': 0, 'gamma': 0, 'lambda': 0, 'alpha': 0}
  bst = xgb.train(params, dtrain)
  </code></pre>
  
  <p>The output includes a long list of statements like</p>
  
  <pre><code>[11:08:18] src/tree/updater_prune.cc:74: tree pruning end, 1 roots, 16 extra nodes, 0 pruned nodes, max_depth=5
  </code></pre>
  
  <p>I've played with several combinations of tuning parameters but I always get this "0 pruned nodes" message.  How can I generate a situation where I get some pruned nodes?</p>