I have a historical table `XY` with these contents:

```
 ID   Person_ID   Balance_on_account   ts
---- ----------- -------------------- ----------
 01   05          +10                  10.10.14
 02   05          -10                  20.10.14
 03   05          -50                  30.10.14
 04   05          +50                  30.10.14
 05   05          -10                  30.10.14
 06   06           11                  11.10.14
 07   06          -40                  15.10.14
 08   06           +5                  16.10.14
 09   06          -10                  30.10.14

```

and I need to create an SQL query which will give me those `Person_ID`'s and timestamps where are

* a) the `Balance_on_account` is negative - that's the easy one,
* b) and at the same time is the record of negative `Balance_on_account` followed by a positive number.

Like for `Person_ID = 05` I would have the row with `ID = 05`, and for `Person_ID = 06` the row with `ID = 09`.