added assignment 3 ex 3.2

src/Assignment3/IntSet.sc

@@ -5,6 +5,8 @@ abstract class IntSet() {
   def union(other: IntSet): IntSet
   def intersect(other: IntSet): IntSet
   def excl(x: Int): IntSet
+  def +(x: Int): IntSet = this.add(x)
+  def -(x: Int): IntSet = this.excl(x)
 }
 
 class NonEmpty(elem: Int, left: IntSet, right: IntSet) extends IntSet() {
@@ -82,3 +84,7 @@ s2.excl(0)
 s2.excl(6)
 s2.excl(2)
 s2.excl(4)
+
+val o1 = Empty + 3 + 4 + 12 + 5
+val o2 = (o1 - 3 - 4)
+o2  // ((-|5|-)|12|-)