For a collection of pairs (k,v), create a map from each k
to the
collection of v
s with which it is associated.
For a collection of pairs (k,v), create a map from each k
to the
collection of v
s with which it is associated.
Equivalent to self.groupBy(_._1).map { case (k, elems) => (k, elems.map(_._2)) }
Map from k
s to collections of v
s