Example of Fold

[Ron Ayoub]

I'm want to fold an RDD into a smaller RDD with max elements. I have simple 
bean objects with 4 properties. I want to group by 3 of the properties and then 
select the max of the 4th. So I believe fold is the appropriate method for 
this. My question is, is there a good fold example out there. Additionally, 
what it the zero value used for as the first argument? Thanks.