fun <T : Any> possiblySequence(element: T?): Sequence<T> =
    if (element == null) emptySequence() else sequenceOf(element)

internal class SingletonSequence<T>(private val element: T) : Sequence<T> {
    override fun iterator(): Iterator<T> = object : Iterator<T> {
        private var hasNext = true
        override fun hasNext(): Boolean = hasNext
        override fun next(): T = if (hasNext) {
            hasNext = false
            element
        } else {
            throw NoSuchElementException()
        }
    }
}

someSequence.mapNotNull { possiblyNullableTransform(it) }