traits.md "shaping up with traits"

[tunesmith]

The solution for Up 2 (Da Streets) didn't quite work for me. Minor note: I'm seeing that the Rectangle case class has "val"'s in the constructor. But as for the error, in Square, redefining width/height as val led to 0.0 results for area and perimeter. Keeping them as def's allowed them to calculate correctly:

@ case class Square(size: Double) extends Rectangular {
    val width = size
    val height = size
  }
defined class Square

@ val sq = Square(3)
sq: Square = Square(3.0)

@ sq.perimeter
res6: Double = 0.0

@ case class Square(size: Double) extends Rectangular {
  def width = size
  def height = size
  }
defined class Square

@ val sq = Square(3)
sq: Square = Square(3.0)

@ sq.perimeter
res9: Double = 12.0

@ sq.area
res10: Double = 9.0