flatten
open fun <T : Collection<out Any>?> flatten(elements: Array<Any>, addTo: T, flattenMaps: Boolean): T
open fun <T : Collection<out Any>?> flatten(elements: Collection<out Any>, addTo: T, flattenMapsAndArrays: Boolean): T
open fun <T : Collection<out Any>?> flatten(elements: Collection<out Any>, addTo: T, flattenMaps: Boolean, flattenArrays: Boolean): T