Return the members of a set resulting from the union of all the sets hold at the specified
keys. Like in {@link #lrange(string, long, long) LRANGE} the result is sent to the client as a
multi-bulk reply (see the protocol specification for more information). If just a single key is
specified, then this command produces the same result as {@link #smembers(string) SMEMBERS}.
<p>
Non existing keys are considered like empty sets.
<p>
Time complexity O(N) where N is the total number of elements in all the provided sets
@param keys
@return Multi bulk reply, specifically the list of common elements.
Return the members of a set resulting from the union of all the sets hold at the specified keys. Like in {@link #lrange(string, long, long) LRANGE} the result is sent to the client as a multi-bulk reply (see the protocol specification for more information). If just a single key is specified, then this command produces the same result as {@link #smembers(string) SMEMBERS}. <p> Non existing keys are considered like empty sets. <p> Time complexity O(N) where N is the total number of elements in all the provided sets @param keys @return Multi bulk reply, specifically the list of common elements.