Use traits

recitation-sessions/recitation-session-3.md

@@ -16,7 +16,7 @@ Note that the above defines a total order.
 Here is the structure we will be using for implementing these trees:
 
 ```scala
-abstract class Tree[T] { ... }
+trait Tree[T] { ... }
 case class EmptyTree[T](leq: (T, T) => Boolean) extends Tree[T] { ... }
 case class Node[T](left: Tree[T], elem: T, right: Tree[T],
     leq: (T, T) => Boolean) extends Tree[T] { ... }
@@ -39,7 +39,7 @@ Given only `leq` for comparison, how can you test for equality? How about strict
 Define the size method on `Tree[T]`, which returns its size, i.e. the number of Nodes in the tree.
 
 ```scala
-abstract class Tree[T] {
+trait Tree[T] {
   def size: Int
   ...
 }