filter

df.filter { age > 18 && name.firstName.startsWith("A") }

val age by column<Int>()
val name by columnGroup()
val firstName by name.column<String>()

df.filter { age() > 18 && firstName().startsWith("A") }
// or
df.filter { it[age] > 18 && it[firstName].startsWith("A") }

df.filter { "age"<Int>() > 18 && "name"["firstName"]<String>().startsWith("A") }