AngularConstraint = function(theta_p, theta) {
            if (arguments.length > 1) {
              this.theta = theta;
              this.rootPos = new Vec2D(theta_p);
            } else {
              this.rootPos = new Vec2D();
              this.theta = theta_p;
            }
            //due to lack-of int/float types, no support of theta in degrees
          }
@param {Vec2D | Number} vector | angle
@param {Number} [theta]