numpy.nditer.remove_multi_index
method
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nditer.remove_multi_index()
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When the “multi_index” flag was specified, this removes it, allowing the internal iteration structure to be optimized further. 
    © 2005–2019 NumPy Developers
Licensed under the 3-clause BSD License.
    https://docs.scipy.org/doc/numpy-1.16.1/reference/generated/numpy.nditer.remove_multi_index.html