foam-extend4.1-coherent-io/applications/solvers/multiphase/compressibleInterFoam/readControls.H

   #include "readPISOControls.H"
   #include "readTimeControls.H"

    nAlphaSubCycles = readLabel(piso.lookup("nAlphaSubCycles"));

    if (nAlphaSubCycles > 1 && nOuterCorr != 1)
    {
        FatalErrorIn(args.executable())
            << "Sub-cycling alpha is only allowed for PISO, "
               "i.e. when the number of outer-correctors = 1"
            << exit(FatalError);
    }