Let's say I have a dataframe with N rows. I want to pick the rows where the row location modulo P gives Q. So for concreteness, let's say P = 7 and Q = 5.

```
Row 0: 0 mod 7 = 0 (not satisfied)
Row 1: 1 mod 7 = 1 (not satisfied)
...
Row 5: 5 mod 7 = 5 (satisfied)
...
Row 12: 12 mod 7 = 5 (satisfied)

```

So the rows that are selected will be 5, 12, 19, 26 ....

If Q=0, you can use the slicing method df.iloc[::P]. How does one do it for mod P = Q?