Add the specified members to the set stored at `key`. Specified members that are
already a member of this set are ignored. If `key` does not exist, a new set is
created before adding the specified members.

An error is returned when the value stored at `key` is not a set.

@return

@integer-reply: the number of elements that were added to the set, not including
all the elements already present into the set.

@history

- `>= 2.4`: Accepts multiple `member` arguments. Redis versions before 2.4 are
  only able to add a single member per call.

@examples

```cli
SADD myset "Hello"
SADD myset "World"
SADD myset "World"
SMEMBERS myset
```