     @type Matrix
     ###
     inverse: ->
-      determinant = this.a * this.d - this.b * this.c
+      determinant = @a * @d - @b * @c
 
       Matrix(
-        this.d / determinant,
-        -this.b / determinant,
-        -this.c / determinant,
-        this.a / determinant,
-        (this.c * this.ty - this.d * this.tx) / determinant,
-        (this.b * this.tx - this.a * this.ty) / determinant
+        @d / determinant,
+        -@b / determinant,
+        -@c / determinant,
+        @a / determinant,
+        (@c * @ty - @d * @tx) / determinant,
+        (@b * @tx - @a * @ty) / determinant
       )
 
     ###*

    @type Matrix
    ###
    rotate: (theta, aboutPoint) ->
      this.concat(Matrix.rotation(theta, aboutPoint))

    ###*
    Returns a new matrix that corresponds this matrix multiplied by a

    ###
    concat: (matrix) ->
      Matrix(
        @a * matrix.a + @c * matrix.b,
        @b * matrix.a + @d * matrix.b,
        @a * matrix.c + @c * matrix.d,
        @b * matrix.c + @d * matrix.d,
        @a * matrix.tx + @c * matrix.ty + @tx,
        @b * matrix.tx + @d * matrix.ty + @ty
      )

    ###*
