How can you establish whether a floating point number is `nan`

or not without using `math.isnan`

or `numpy.isnan`

?

We cannot compare with `==`

since `nan`

is not equal to itself. However, it is the only floating point number which is not equal to itself, so use `!=`

instead:

```
In [x]: c = 0 * 1.e1000 # 0 * inf is nan
In [x]: c != c
Out[x]: True # c isn't equal to itself, so must be nan
```