Function union`Const`

union<T>(
seq: Iterable<T>,
setFactory?: SetFactory<T>,
): (src: Iterable) => Iterable
Produces the union of two iterables, preserving the first occurrence of each distinct element.

Curried version of _union.
Type Parameters
- T
  Element type produced by the source and additional iterables.
Parameters
- seq: Iterable<T>
  Secondary iterable whose unique elements extend the union.
- OptionalsetFactory: SetFactory<T>
  Optional factory creating the set instance used for uniqueness tracking.
Returns (src: Iterable) => Iterable
A deferred iterable yielding distinct elements from src followed by unseen items from seq.
Throws
Error Rethrows any error thrown by setFactory or during enumeration of the inputs.
Example
```
const left = [1, 2, 3];
const right = [3, 4, 5];
const merged = [..._union(left, right)];
console.log(merged); // [1, 2, 3, 4, 5]
```
or using the curried version:
```
const merged = [
  ...pipeInto(
    [1, 2, 3],
    union([3, 4, 5])
  ),
];
console.log(merged); // [1, 2, 3, 4, 5]
```