`CCMixset`

Set of Heterogeneous Values

```
let k1 : int key = newkey () in
let k2 : int key = newkey () in
let k3 : string key = newkey () in
let set =
empty
|> set ~key:k1 1
|> set ~key:k2 2
|> set ~key:k3 "3"
in
assert (get ~key:k1 set = Some 1);
assert (get ~key:k2 set = Some 2);
assert (get ~key:k3 set = Some "3");
()
```

`val newkey : unit -> 'a key`

`newkey ()`

creates a new unique key that can be used to access a `'a`

value in a set. Each key created with `newkey`

is distinct from any other key, even if they have the same type.

Not thread-safe.

`val empty : t`

Empty set.

`set ~key v set`

maps `key`

to `v`

in `set`

. It means that for every `set`

, `get ~key (set ~key v set) = Some v`

.

`val cardinal : t -> int`

Number of mappings.