code: function(x, y) {
          return Math.atan2(
              this.x * y - this.y * x,
              this.x * x + this.y * y);
      }
docs: /*
  Find the angle of the two vectors, solving the formula for
  the cross product a x b = |a||b|sin(θ) for θ.
  @param {Number} x the x-coordinate
  @param {Number} y the y-coordinate
  @return {Number} the angle in radians