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

    Constraint.call(this, options);

    this.impulse = 0;
    this.distance = 0;
    this.normal = new Vec3();
    this.velocityBias = 0;
    this.divisor = 0;
}
A constraint that maintains the direction of one body from another.

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