title: Concatenate multiple pandas series efficiently
body: <p>I understand that I can use <code>combine_first</code> to merge two series:</p>
  
  <pre><code>series1 = pd.Series([1,2,3,4,5],index=['a','b','c','d','e'])
  series2 = pd.Series([1,2,3,4,5],index=['f','g','h','i','j'])
  series3 = pd.Series([1,2,3,4,5],index=['k','l','m','n','o'])
  
  Combine1 = series1.combine_first(series2)
  print(Combine1
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>a    1.0
  b    2.0
  c    3.0
  d    4.0
  e    5.0
  f    1.0
  g    2.0
  h    3.0
  i    4.0
  j    5.0
  dtype: float64
  </code></pre>
  
  <p>What if I need to merge 3 or more series?</p>
  
  <p>I understand that using the following code: <code>print(series1 + series2 + series3)</code>yields:</p>
  
  <pre><code>a   NaN
  b   NaN
  c   NaN
  d   NaN
  e   NaN
  f   NaN
  ...
  dtype: float64
  </code></pre>
  
  <p>Can I merge multiple series efficiently without using <code>combine_first</code> multiple times?</p>
  
  <p>Thanks</p>