code: function angle(v) {
           return Math.acos(me.Math.clamp(this.dotProduct(v) / (this.length() * v.length()), -1, 1));
         }
docs: return the angle between this vector and the passed one
  @name angle
  @memberOf me.Vector2d
  @function
  @param {me.Vector2d} v
  @return {Number} angle in radians