IntSet from Functional Programming Coursera. Help me understand the union function!

gistfile1.scala

abstract class IntSet {
  def incl(x: Int): IntSet
  def contains(x: Int): Boolean
  def union(other: IntSet): IntSet
}

class Empty extends IntSet {
  def incl(x: Int): IntSet = new NonEmpty(x, new Empty, new Empty)
  def contains(x: Int): Boolean = false
  override def toString = "."
  def union(other: IntSet): IntSet = other
}

class NonEmpty(elem: Int, left: IntSet, right: IntSet) extends IntSet {
  def incl(x: Int): IntSet = 
    if ( x > elem )      new NonEmpty( elem, left, right incl x )
    else if ( x < elem ) new NonEmpty( elem, left incl x, right )
    else this

  def contains( x: Int ): Boolean = 
    if ( x < elem ) left contains x
    else if ( x > elem ) right contains x 
    else true
  override def toString = "{" + left + elem + right + "}"

  def union(other: IntSet): IntSet = 
    ((left union right) union other) incl elem

}