public OneWayOneDOFConstraint(TargetVelocityManifold m, Float3 M_inv, float dt)
    {
        e  = m.e;
        id = m.id;

        this.M_inv = M_inv;

        constraint = new OneDOFConstraint <Float3>(
            J: new float3(m.normal, Lin.Cross(m.r, m.normal)),
            M_inv: M_inv,
            bias: -m.targetSpeed,
            softness: m.softness
            );

        lambdaAccum = 0;
    }
 public OneWayOneDOFConstraint(TargetVelocityManifold m, ComponentDataFromEntity <Mass> masses, float dt) :
     this(m, masses[m.e].M_inv, dt)
 {
 }