The GenMapFactory class has an
def apply[A, B](elems: (A, B)*): CC[A, B] = (newBuilder[A, B] ++= elems).result()
The * indicates a variable length argument list (also known as varargs).
In the case of a Map you would use it like this:
val newMap: Map[A, B] = Map((a0, b0), (a1, b1), (a2, b2))
There is also a neater way to express the (key, value) syntax, like this:
val newMap: Map[A, B] = Map(a0 -> b0, a1 -> b1, a2 -> b2)