command_builder = rom.command   # no relation given, so get builder
nested_command = command_builder.create(user: :users) do |user|
  user.create(:books)
end

# equivalent:
nested_command = rom.command({user: :users}, [:create, [:books, [:create]]])

# equivalent
user_create = rom.command(:users)[:create]
books_create = rom.comand(:books)[:create]
nested_command = -> input {
  user_create.call(input[:user]).combine(
    books_create.call(input[:user][:books])
  )
}

# and books_create command is built to take (books, user)
# and gets partially applied with :books, so on combine
# it receives output of user_create command, which will be a user