of the two
input arrays.

Parameters
----------
ar1, ar2 : array_like
    Input arrays. They are flattened if they are not already 1D.

Returns
-------
union1d : ndarray
    Unique, sorted union of the input arrays.

Examples
--------
>>> import numpy as np
>>> np.union1d([-1, 0, 1], [-2, 0, 2])
array([-2, -1,  0,  1,  2])

To find the union of more than two arrays, use functools.reduce:

>>> from functools import reduce
>>> reduce(np.union1d, ([1, 3, 4, 3], [3, 1, 2, 1], [6, 3, 4, 2]))
array([1, 2, 3, 4, 6])
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