singleOrNull

Returns single element, or null if the sequence is empty or has more than one element.

fun <T> Sequence<T>.singleOrNull(): T?(source)

fun main() {
    val singleValue   = sequenceOf("only").singleOrNull()      // → "only"
    val emptySequence = emptySequence<Int>().singleOrNull()    // → null
    val manyValues    = sequenceOf(1, 2).singleOrNull()        // → null

    println(singleValue)   // prints: only
    println(emptySequence) // prints: null
    println(manyValues)    // prints: null
}

Source