Snippet that defines an empty mutable 2D list, adds values to it, and then converts it into a fully immutable 2D list.

2DMutableToImmutable.kt

fun main() {
    // 1. Define an empty mutable 2D list
    val twoDList = mutableListOf<MutableList<Int>>()
    println("1. Initial empty mutable list: $twoDList")

    // 2. Add inner lists to it later
    twoDList.add(mutableListOf(10, 20, 30))
    twoDList.add(mutableListOf(40, 50, 60))
    println("2. Mutable list after adding rows: $twoDList")

    // You can still modify it
    twoDList[0].add(99)
    println("3. After modifying an inner list: $twoDList")

    // 4. Convert the final structure to an immutable 2D list
    val immutable2DList: List<List<Int>> = twoDList.map { it.toList() }
    println("4. Final immutable list: $immutable2DList")
    
    // Any attempt to modify immutable2DList will now result in a compile error.
}