Add the specified member to the set value stored at key. If member is already a member of the
set no operation is performed. If key does not exist a new set with the specified member as
sole member is created. If the key exists but does not hold a set value an error is returned.
<p>
Time complexity O(1)
@param key
@param members
@return Integer reply, specifically: 1 if the new element was added 0 if the element was
already a member of the set
Add the specified member to the set value stored at key. If member is already a member of the set no operation is performed. If key does not exist a new set with the specified member as sole member is created. If the key exists but does not hold a set value an error is returned. <p> Time complexity O(1) @param key @param members @return Integer reply, specifically: 1 if the new element was added 0 if the element was already a member of the set