filter unique values conditional on groups : I have the following table:

`id  L1  L2
1   A   B
1   A   C
1   A   D
1   B   B
1   B   C
1   B   D
2   D   A
2   D   F`

I want to filter out any L2 value that's in L1, but only within same id group. So for id=1, we filter out L2=B, but keep L2=D. For id=2, we keep L2=A.

Output should be like:

 `id  L1  L2
1   A   C
1   A   D
1   B   C
1   B   D
2   D   A
2   D   F`