val list = """
        a,234,97654/
        b,23,ABC,
        b,44235,203
        a,234,97654/
        b,23,ABC
        b,44235,203
        49,234
        39,9291
    """.trimIndent().split("\n")

fun <A> List<A>.splitBy(pred : (A) -> Boolean) : List<List<A>> {
    if(size == 0) return listOf()
    val loc = indexOfLast(pred)
    if(loc == -1) return listOf(this)
    return subList(0, loc).splitBy(pred) + listOf(subList(loc, size))
}

fun cond(s : String) : Boolean = s.startsWith("a") && s.endsWith("/")

list.splitBy(::cond)
    .map { l->
        l.takeWhile { s ->
            s.startsWith("b") || cond(s)
        }.joinToString("")
    }