Extension method that returns the combinations of the elements of a list

Combinations.kt

/**
 * Returns the combinations of the elements in this list as a list of pairs.
 *
 * The returned list is empty if this collection contains less than two elements.
 *
 * @return the combinations of the elements
 */
fun <E>List<E>.combine(): List<Pair<E, E>> = this.foldIndexed(mutableListOf()) { i, acc, element ->

  for (j in i + 1 until this.size) {
    acc.add(Pair(element, this[j]))
  }

  acc
}