     @type Matrix
     ###
     scale: (sx, sy, aboutPoint) ->
-      this.concat(Matrix.scale(sx, sy, aboutPoint))
+      @concat(Matrix.scale(sx, sy, aboutPoint))
 
     ###*
     Returns the result of applying the geometric transformation represented by the 

    ###
    transformPoint: (point) ->
      Point(
        this.a * point.x + this.c * point.y + this.tx,
        this.b * point.x + this.d * point.y + this.ty
      )

    ###*

    @type Matrix
    ###
    inverse: ->
      determinant = @a * @d - @b * @c

      Matrix(
        @d / determinant,
        -@b / determinant,
        -@c / determinant,
        @a / determinant,
        (@c * @ty - @d * @tx) / determinant,
        (@b * @tx - @a * @ty) / determinant
      )

    ###*
