title: Find indices of elements equal to zero in a NumPy array
body: <p>NumPy has the efficient function/method <a href="http://docs.scipy.org/doc/numpy/reference/generated/numpy.nonzero.html" rel="noreferrer"><code>nonzero()</code></a> to identify the indices of non-zero elements in an <code>ndarray</code> object. What is the most efficient way to obtain the indices of the elements that <em>do</em> have a value of zero?</p>