Why do I get a "error: not found: type LineElement" when trying to load this scala file?

gistfile1.txt

abstract class Element {
  def contents: Array[String]
  def height: Int = contents.length
  def width: Int = if (height == 0) 0 else contents(0).length
  
  def beside(that: Element): Element = {
    new LineElement("hello")
  }
}


class ArrayElement(conts: Array[String]) extends Element  {
  def contents: Array[String] = conts
}

class LineElement(s:String) extends Element() {
  val contents = Array(s)
  override def width = s.length
  override def height = 1
}