flatMapTo

Appends all elements yielded from results of transform function being invoked on each character of original char sequence, to the given destination.

@IgnorableReturnValueinline fun <R, C : MutableCollection<in R>> CharSequence.flatMapTo(destination: C, transform: (Char) -> Iterable<R>): C(source)

fun main() {
    val input: CharSequence = "abc"
    val destination = mutableListOf<String>()

    input.flatMapTo(destination) { ch ->
        // For each character, emit the character itself and its uppercase variant
        listOf(ch.toString(), ch.uppercaseChar().toString())
    }

    println(destination)   // prints: [a, A, b, B, c, C]
}

Source