title: Combine 2 series pandas - overwriting the NANs
body: <p>If we have 2 series: </p>
  
  <pre><code>a = pd.Series([1, 2, 3, np.nan, 5, np.nan, np.nan])
  b = pd.Series([np.nan, np.nan, np.nan, 4, np.nan, 6, 7])
  </code></pre>
  
  <p>How can I get c, such that</p>
  
  <pre><code>a                 b             c
  ========          ========      =========
  0    1.0          0    NaN      0    1.0     
  1    2.0          1    NaN      1    2.0
  2    3.0          2    NaN      2    3.0
  3    NaN          3    4.0      3    4.0
  4    5.0          4    NaN      4    5.0
  5    NaN          5    6.0      5    6.0
  6    NaN          6    7.0      6    7.0
  </code></pre>
  
  <p>where c is the overlap of a &amp; b, if the value is not a NaN (it will always be the case)</p>