title: What does the c underscore expression `c_` do exactly?
body: <p>It seems to be some kind of horizontal concatenation, but I could not find any documentation online. Here a minimal working example: </p>
  
  <pre><code>In [1]: from numpy import c_
  In [2]: a = ones(4)
  In [3]: b = zeros((4,10))    
  In [4]: c_[a,b]
  Out[4]: 
  array([[ 1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
         [ 1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
         [ 1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
         [ 1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.]])
  </code></pre>