# Set Specific Operations

The Kotlin collections package contains extension functions for popular operations on sets: finding intersections, merging, or subtracting collections from each other.

To merge two collections into one, use the `union()`

function. It can be used in the infix form `a union b`

.
Note that for ordered collections the order of the operands is important: in the resulting collection, the elements of the first operand go before the elements of the second.

To find an intersection between two collections (elements present in both of them), use `intersect()`

.
To find collection elements not present in another collection, use `subtract()`

.
Both these functions can be called in the infix form as well, for example, `a intersect b`

.

```
fun main() {
//sampleStart
val numbers = setOf("one", "two", "three")
println(numbers union setOf("four", "five"))
println(setOf("four", "five") union numbers)
println(numbers intersect setOf("two", "one"))
println(numbers subtract setOf("three", "four"))
println(numbers subtract setOf("four", "three")) // same output
//sampleEnd
}
```

Note that set operations are supported by `List`

as well.
However, the result of set operations on lists is still a `Set`

, so all the duplicate elements are removed.