c# - Conditional filtering for grouping

Question

Not sure if the question is correct, as I'm having some trouble with what the term is for what I'm looking for exactly. Have the following data:

int? Sequence
Date theDate
int? anID

What I need to do is apply the following rules to this sequence (LINQ):

• no 2 elements which have a Sequence have the same non-null anIDs.
• no 2 elements have the same theDate and anIDs if one of them has a Sequence.
• element N’s theDate is more recent than element N-1s theDate.
• element N’s theDate = element N-1s theDate AND element N's Sequence > element N-1's Sequence.

How do I manage this? Do I need to use nested group by's? Will this be conditional group by's?

Am a little lost on my implementation and terminology and will update question if I get a better understanding of the solution.