Welcome to Scala version 2.10.0-M4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.collection.immutable.SortedSet
import scala.collection.immutable.SortedSet

scala> class Foo(val name: String, val n: Int) {
     | override def equals(obj: Any): Boolean = obj match { case other: Foo => name == other.name; case _ => false }
     | override def hashCode = name.##
     | override def toString = "Foo(" + name + ", " + n + ")"
     | }
defined class Foo

scala> implicit val ordering: Ordering[Foo] = Ordering.fromLessThan[Foo] { (a, b) => a.name.compareTo(b.name) < 0 }
ordering: Ordering[Foo] = scala.math.Ordering$$anon$9@32370bf9

scala> SortedSet(new Foo("bar", 1)) + new Foo("bar", 2)
res0: scala.collection.immutable.SortedSet[Foo] = TreeSet(Foo(bar, 2))

Welcome to Scala version 2.10.0-M4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_31).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import scala.collection.immutable.TreeSet
import scala.collection.immutable.TreeSet

scala> import scala.collection.immutable.SortedSet
import scala.collection.immutable.SortedSet

scala> val s: SortedSet[String] = TreeSet()
s: scala.collection.immutable.SortedSet[String] = TreeSet()

scala> s + new String("foo")
res0: scala.collection.immutable.SortedSet[String] = TreeSet(foo)

scala> res0.head.hashCode
res1: Int = 101574

scala> System.identityHashCode(res0.head)
res2: Int = 1965496749

scala> System.identityHashCode(res0 + new String("foo"))
res3: Int = 856318696

scala> System.identityHashCode(res0 + new String("foo"))
res4: Int = 143836998

scala> System.identityHashCode((res0 + new String("foo")).head)
res5: Int = 1652856443