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+Computes the union of `numkeys` sorted sets given by the specified keys, and
+stores the result in `destination`. It is mandatory to provide the number of
+input keys (`numkeys`) before passing the input keys and the other (optional)
+arguments.
+
+By default, the resulting score of an element is the sum of its scores in the
+sorted sets where it exists.
+
+Using the `WEIGHTS` option, it is possible to specify a multiplication factor
+for each input sorted set. This means that the score of every element in every
+input sorted set is multiplied by this factor before being passed to the
+aggregation function. When `WEIGHTS` is not given, the multiplication factors
+default to `1`.
+
+With the `AGGREGATE` option, it is possible to specify how the results of the
+union are aggregated. This option defaults to `SUM`, where the score of an
+element is summed across the inputs where it exists. When this option is set to
+either `MIN` or `MAX`, the resulting set will contain the minimum or maximum
+score of an element across the inputs where it exists.
+
+If `destination` already exists, it is overwritten.
+
+@return
+
+@integer-reply: the number of elements in the resulting sorted set at
+`destination`.
+
+@examples
+
+```cli
+ZADD zset1 1 "one"
+ZADD zset1 2 "two"
+ZADD zset2 1 "one"
+ZADD zset2 2 "two"
+ZADD zset2 3 "three"
+ZUNIONSTORE out 2 zset1 zset2 WEIGHTS 2 3
+ZRANGE out 0 -1 WITHSCORES
+```