joferkington/numpy — comment on commit 408f94c2acebbd0339914d71f0b7248dcbcaf940
File: numpy/core/numeric.py line 2105
By charris:

You can simplify this a bit since the maskna property propagates:

In [2]: a = arange(5, maskna=1)

In [3]: b = arange(5, maskna=0)

In [4]: a*b
Out[4]: array([ 0, 1, 4, 9, 16], maskna=True)