impl<E, N: Dim, D: DualNum<f64> + Copy> State<E, N, D>
where
    DefaultAllocator: Allocator<N>,
{
    /// Total moles $N=\sum_iN_i$
    pub fn total_moles(&self) -> FeosResult<Moles<D>> {
        self.total_moles.ok_or(FeosError::IntensiveState)
    }
}

pub trait Composition<D: DualNum<f64> + Copy, N: Dim>
where
    DefaultAllocator: Allocator<N>,
{
    fn into_molefracs<E: Residual<N, D>>(self, eos: &E) -> (OVector<D, N>, Option<Moles<D>>);
}

    // every constructor goes through this private function
    fn _new(
        eos: &E,
        temperature: Temperature<D>,
        density: Density<D>,
        molefracs: OVector<D, N>,
        total_moles: Option<Moles<D>>,
    ) -> FeosResult<Self>;

    // the basic extensive constructor
    pub fn new_nvt<X: Composition<D, N>>(
        eos: &E,
        temperature: Temperature<D>,
        volume: Volume<D>,
        composition: X,
    ) -> FeosResult<Self>;

    // the basic intensive constructor
    pub fn new<X: Composition<D, N>>(
        eos: &E,
        temperature: Temperature<D>,
        density: Density<D>,
        composition: X,
    ) -> FeosResult<Self>;

    // some small helper functions for special cases
    pub fn new_density(
        eos: &E,
        temperature: Temperature<D>,
        partial_density: Density<OVector<D, N>>,
    ) -> FeosResult<Self>;
    pub fn new_pure(eos: &E, temperature: Temperature<D>, density: Density<D>) -> FeosResult<Self>;

    // the pressure constructors
    pub fn new_npt<X: Composition<D, N>>(
        eos: &E,
        temperature: Temperature<D>,
        pressure: Pressure<D>,
        composition: X,
        density_initialization: Option<DensityInitialization>,
    ) -> FeosResult<Self>;
    pub fn new_tpvx(
        eos: &E,
        temperature: Temperature<D>,
        pressure: Pressure<D>,
        volume: Volume<D>,
        molefracs: OVector<D, N>,
        density_initialization: Option<DensityInitialization>,
    ) -> FeosResult<Self>;
    
    // the caloric constructors (unchanged)
    pub fn new_nph(...)
    pub fn new_nth(...)
    pub fn new_nps(...)
    pub fn new_nts(...)
    pub fn new_nvu(...)
    
    // the builder constructors
    pub fn build<X: Composition<D, N>>(
        eos: &E,
        temperature: Temperature<D>,
        volume: Option<Volume<D>>,
        density: Option<Density<D>>,
        composition: X,
        pressure: Option<Pressure<D>>,
        density_initialization: Option<DensityInitialization>,
    ) -> FeosResult<Self>;
    pub fn build_full<X: Composition<D, N> + Clone>(
        eos: &E,
        temperature: Option<Temperature<D>>,
        volume: Option<Volume<D>>,
        density: Option<Density<D>>,
        composition: X,
        pressure: Option<Pressure<D>>,
        molar_enthalpy: Option<MolarEnergy<D>>,
        molar_entropy: Option<MolarEntropy<D>>,
        molar_internal_energy: Option<MolarEnergy<D>>,
        density_initialization: Option<DensityInitialization>,
        initial_temperature: Option<Temperature<D>>,
    ) -> FeosResult<Self>;

pub struct PhaseEquilibrium<E, const P: usize, N: Dim = Dyn, D: DualNum<f64> + Copy = f64>
where
    DefaultAllocator: Allocator<N>,
{
    states: [State<E, N, D>; P],
    pub phase_fractions: [D; P],
    total_moles: Option<Moles<D>>,
}