Creating a "bag" in Scala with foldLeft

Bag.scala

// Given a list of items, return a Map[A -> Int] where A is an element and Int is how many times it appears
val result = List("a", "b", "a").foldLeft(Map[String, Int]())( (acc, el) =>
                if (acc.contains(el) ) {
                    val x = acc.get(el).get
                   acc + (el -> (x+1))
                }
                else { acc + (el -> 1) } )

assert(result == Map("a" -> 2, "b" -> 1)

Note - could've used acc(el) on line 4 per this post - http://stackoverflow.com/questions/19969225/getting-map-keys-value-cleanly

scala> Map[String, Int]().withDefaultValue(0)
res25: scala.collection.immutable.Map[String,Int] = Map()

scala> List("a","b","a").foldLeft(res25) { (acc, el) => acc + (el -> (acc(el) + 1)) }
res27: scala.collection.immutable.Map[String,Int] = Map(a -> 2, b -> 1)