A.33 library(pairs): Operations on key-value lists
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Availability::- use_module(library(pairs)).(can be autoloaded)
Sourcemap_list_to_pairs(:Function, +List, -Keyed)
Create a Key-Value list by mapping each element of List. For example, if we have a list of lists we can create a list of Length-List using 
        map_list_to_pairs(length, ListOfLists, Pairs),