 slice.

The shape of the return value corresponds to ``keepdims=True``.

Examples
--------
>>> import numpy as np
>>> nan = np.nan
>>> a = np.array([[ 3.,  3., nan,  3.],
                  [ 1., nan,  2.,  4.],
                  [nan, nan,  9., -1.],
                  [nan,  5.,  4.,  3.],
                  [ 2.,  2.,  2.,  2.],
                  [nan, nan, nan, nan]])
>>> _first_nonnan(a, axis=0)
array([[3., 3., 2., 3.]])
>>> _first_nonnan(a, axis=1)
array([[ 3.],
       [ 1.],
       [ 9.],
       [ 5.],
       [ 2.],
       [nan]])
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