code: function rotate(angle) {
           if (angle !== 0) {
             var a = this.val,
                 a0 = a[0],
                 a1 = a[1],
                 a3 = a[3],
                 a4 = a[4],
                 s = Math.sin(angle),
                 c = Math.cos(angle);
             a[0] = a0 * c + a3 * s;
             a[1] = a1 * c + a4 * s;
             a[3] = a0 * -s + a3 * c;
             a[4] = a1 * -s + a4 * c;
           }
  
           return this;
         }
docs: rotate the matrix (counter-clockwise) by the specified angle (in radians).
  @name rotate
  @memberOf me.Matrix2d
  @function
  @param {Number} angle Rotation angle in radians.
  @return {me.Matrix2d} Reference to this object for method chaining