/usr/local/lib/swipl/library/rbtrees.pl
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 rb_partial_map(+Tree, +Keys, :G, -NewTree)
For all nodes Key in Keys, if the value associated with key Key is Val0 in tree Tree, and if call(G,Val0,ValF) holds, then the value associated with Key in NewTree is ValF, otherwise it is the value associated with the key in Tree. Fails if Key isn't in Tree or if call(G,Val0,ValF) is not satisfiable for all Val0 in Keys. Assumes keys are sorted and not repeated (fails if this is not true).