gistfile1.scala

import scalaz.Monoid
import scala.language.higherKinds

import scalaz.syntax.monoid._
import scalaz.std.set._

implicit def mm[K, T, M[_]](implicit mo: Monoid[M[T]]): Monoid[Map[K, M[T]]] = new Monoid[Map[K, M[T]]] {
  def zero = Map.empty
  def append(m1: Map[K, M[T]], m2: => Map[K, M[T]]) = {
    val mMin = List(m1, m2).minBy(_.size)
    mMin.foldLeft(m2){ case (acc, (k, vs)) =>
    	acc.get(k) match {
      	case Some(vs2) => acc.updated(k, mo.append(vs, vs2))
        case None => acc + (k -> vs)
      }
    }
  }
}

Map(1 -> Set(1, 2)) |+| Map(1 -> Set(3, 4)) |+| Map(1 -> Set(5, 6)) // Map(1 -> Set(5, 1, 6, 2, 3, 4))