kotlin.sequences.singleOrNull

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

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

inline fun <T> Sequence<T>.singleOrNull(
    predicate: (T) -> Boolean
): T?

Returns the single element matching the given predicate, or null if element was not found or more than one element was found.