List element via partition

Adding the example that we discussed today for list elements.
It is ***not*** safe.

For `x:a`, `xs : [a]`, if I partition `xs` so that
```
ls :: [{v:a|v=x}]
rs :: [{v:a|v!=x}]
ls cup rs = xs
```
`ls` being empty is not enough to prove that `x` is not an element of `xs`