function Curve(targets, options) {
    if (targets) {
        if (targets instanceof Array) this.targets = targets;
        else this.targets = [targets];
    }
    else this.targets = [];

    Constraint.call(this, options);

    this.impulses = {};
    this.normals = {};
    this.velocityBiases = {};
    this.divisors = {};
}
A constraint that keeps a physics body on a given implicit curve.

@class Curve
@extends Constraint
@param {Particle[]} targets The bodies to track.
@param {Object} options The options hash.