number of elements by which to extend `x` at each end of the axis.
axis : int, optional
    The axis along which to extend `x`. Default is -1.

Examples
--------
>>> import numpy as np
>>> from scipy.signal._arraytools import even_ext
>>> a = np.array([[1, 2, 3, 4, 5], [0, 1, 4, 9, 16]])
>>> even_ext(a, 2)
array([[ 3,  2,  1,  2,  3,  4,  5,  4,  3],
       [ 4,  1,  0,  1,  4,  9, 16,  9,  4]])

Even extension is a "mirror image" at the boundaries of the original array:

>>> t = np.linspace(0, 1.5, 100)
>>> a = 0.9 * np.sin(2 * np.pi * t**2)
>>> b = even_ext(a, 40)
>>> import matplotlib.pyplot as plt
>>> plt.plot(np.arange(-40, 140), b, 'b', lw=1, label='even extension')
>>> plt.plot(np.arange(100), a, 'r', lw=2, label='original')
>>> plt.legend(loc='best')
>>> plt.show()
r