plus

abstract operator fun plus(others: Map<String, C>): Self
open operator fun plus(other: Pair<String, C>): Self
open operator fun plus(others: Iterable<Pair<String, C>>): Self
open operator fun plus(others: Sequence<Pair<String, C>>): Self
open fun plus(first: Pair<String, C>, vararg others: Pair<String, C>): Self