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1) You are able to use any structure there. But you need to have an array (is used in the collepsible block) with two columns:
tite and array of the listitem
2) Two arrays
3) Please use a mapping with a Storage variable with a structure above, like: 
Thanks Sergiy. Very nice, detailed answer. Getting very close, I imagine.
You said earlier, "use a loop by these arrays with checking ids, if they are the same - add information from one array's item to another one." I'm at a loss on how to get these 2 arrays. Using my data structure and your mapping sample, can you show me how to populate the 2 arrays to be used, just so I'm not beating around the bush? Thanks.
You have to add a loop by Years collection and set the Storage variable "collapsible " from the previous message:
each item in the collapsible variable has a title = year and listitems = ids of that year.
Unfortunately this is something outside the scope of standard Appery.io platform support. You may consider purchasing Advisory Pack to get more in-depth help on this question. Here is more information about Advisory Pack (http://appery.io/services/#Advisory_Pack).