# 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`

.

You can also apply `union`

, `intersect`

, and `subtract`

to `List`

. However, their result is *always* a `Set`

, even on lists. In this result, all the duplicate elements are merged into one and the index access is not available.