merge

fun <T> DataFrame<T>.merge(vararg columns: String): Merge<T, Any?, List<Any?>>

inline fun <T, reified C> DataFrame<T>.~~merge~~(vararg columns: ColumnReference<C>): Merge<T, C, List<C>>

inline fun <T, reified C> DataFrame<T>.~~merge~~(vararg columns: KProperty<C>): Merge<T, C, List<C>>

It's recommended to migrate to the String or Extension Properties Access API https://kotlin.github.io/dataframe/apilevels.html