What does this receiver-based builder print?

class LabelBuilder {
    var prefix: String = ""
    private val values = mutableListOf<String>()

    fun value(text: String) {
        values += text
    }

    fun build(): String = values.joinToString(",", "$prefix[", "]")
}

fun labels(block: LabelBuilder.() -> Unit): String =
    LabelBuilder().apply(block).build()

fun main() {
    println(labels {
        prefix = "v"
        value("a")
        value("b")
    })
}