/usr/local/lib/swipl/library/hashtable.pl
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 ht_put(!HT, +Key, +Value, +IfNew, -Old) is det
Add Key-Value to HT. Old is unified with the old value associated with Key or, if Key is new, with IfNew. This can be used to bootstrap managing a list of values, e.g. 
ht_put_list(HT, Key, Value) :-
    ht_put(HT, Key, [Value|Tail], [], Tail).