function Angle(a, b, options) {
    this.a = a;
    this.b = b;

    Constraint.call(this, options);

    this.effectiveInertia = new Mat33();
    this.angularImpulse = new Vec3();
    this.error = 0;
}
A constraint that keeps a physics body a given direction away from a given
 anchor, or another attached body.

 @class Angle
 @extends Constraint
 @param {Particle} a One of the bodies.
 @param {Particle} b The other body.
 @param {Object} options An object of configurable options.