pandas.DatetimeIndex.union_many
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DatetimeIndex.union_many(others)[source]
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A bit of a hack to accelerate unioning a collection of indexes 
    © 2008–2012, AQR Capital Management, LLC, Lambda Foundry, Inc. and PyData Development Team
Licensed under the 3-clause BSD License.
    https://pandas.pydata.org/pandas-docs/version/0.20.3/generated/pandas.DatetimeIndex.union_many.html