Relational Algebra Expression

Hi,

I've been trying to figure out how to use relational algebra to
extract some data from the following relation:
<Year, Name, Votes> where the primary key is Year+Name (composite
key). A sample table could contain the following entries:
1999 | Joe | 34
1999 | James | 45
2000 | Mike | 24
2001 | George | 17
2001 | Josh | 69

What I'm trying to do is extract all the years where we have more than
1 relation for the year. For the sample table, we need to get:
1999
2001

I tried to do in a couple of ways but I couldn't. I tried using a
projection on Year so that I can remove duplicates but then I can't
just use difference because the new relations are not compatible. Any
hints are greatly appreciated.

Thanks,
g
.

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