Perform operations on list elements from coll

I'm a bit stuck on this. I have a [coll] that is outputting lists of integer pairs (eg. index, val1 val2). I would like to perform an addition on just the first element of the lists (index, (val1+x) val2).

In addition, I would like to perform the calculation only on values that are greater than a given index, so if given an index 1 for a coll containing four lists (0-based), the operation is only calculated on lists indexed 2 and 3.

Finally, I need to join them back up with their associated index.

I have a semi functional patch that just performs the operation on each individual index and downstream selects whether or not it will pass the processed list or the unprocessed lists according to a given index, before collecting it all back into another [coll]. It is pretty clunky though and because the selected index is changing, the indexed data can get a bit scrambled up. I can't help but think there's a more robust solution.

The data is for writing to a [function], essentially mimicking the behaviour of the mouse mode "shift", but for numeric input instead of mousing.