If the mapped keys
* may have duplicates, use {@link #toMap(Function, Function, BinaryOperator)}...For parallel stream
* pipelines, the {@code combiner} function operates by merging the keys...static factory methods of(Supplier, BiConsumer, BinaryOperator, Characteristics...) can be used to construct...This rules out, for example, "forked" streams, where the same source feeds two or more pipelines, or...Stream pipelines may execute either sequentially or in parallel.