Here's an idea.
Provide some example data. You need not provide all 50 variables but 3 or 4 should be sufficient to show the procees.
Then show what the final result should look like for that data.
I have a feeling that you are making this way more complicated than it should be.
NOTE that First. and Last. refer to the ROW order in the data set. Also the value of First. is either 1 or 0 for true (the record is the first fo the specified group) or false (the record is not the first of the group).
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