void billingEngine_EinvReadingQuantity(object sender, MercatorUi.Engine.Gescom.BillingEngine.EinvReadingValueEventArgs<double, double?> e)
{
    if (e.DrLigne.Value<double>("free_coeff") != 0)
        e.ChangedValue = Math.Round(e.DrLigne.Value<double>("q") * e.DrLigne.Value<double>("free_coeff"), Globals.N_DEC_Q);
}