Kotlin 1.3.0 allows you to mark the
main() function with
suspend. Kotlin will
wrap it in a suitable coroutine builder, and you can call other
main() without your own coroutine builder.
Most of the samples on this site will use a manually-created coroutine builder,
though. While everything in the Klassbook centers around
main(), very little "real world"
code does, and your choice of coroutine builder and coroutine scope are fairly