Since mpnc uses fugacities, users have to use a constraint solver to compute them from whatever initial conditions they want to have in terms of saturations, mole fractions, etc... If one wants single-phasic initial conditions (without dissolved components), and one does for instance: ```cpp GetPropType<TypeTag, Properties::FluidState> fs; // ... set stuff in fluid state -> I want single-phasic state // use constraint solver to compute fugacities using MiscibleMultiPhaseComposition = Dumux::MiscibleMultiPhaseComposition<Scalar, FluidSystem>; typename FluidSystem::ParameterCache paramCache; MiscibleMultiPhaseComposition::solve(fs, paramCache); ``` one gets already nonzero mole fractions of the non-present phases in the initially present phase, because `MiscibleMultiPhaseComposition` seems to assume that all phases are present. One therefore has to use `ComputeFromReferencePhase` in this case, which to me does not seem obvious. We should maybe either rethink our assumptions in `MiscibleMultiPhaseComposition`, or give the user some hints/warnings/info... This could be achieved by adding an "impl" layer which we use internally, while `MiscibleMultiPhaseComposition`, etc. forward to the impl but give some descriptive information message. For instance: "This uses the assumption that all phases are present. For single-phase situations, use ..."