cdm/src/main/java/ucar/nc2/ft2/coverage/Time2DOffsetCoordSys.java
public Optional<List<CoverageCoordAxis>> subset(SubsetParams params, AtomicBoolean isConstantForcast, boolean makeCFcompliant) {
    List<CoverageCoordAxis> result = new ArrayList<>();

    Optional<CoverageCoordAxis> axiso = runAxis.subset(params);
    if (!axiso.isPresent())
      return Optional.empty(axiso.getErrorMessage());
    CoverageCoordAxis1D runAxisSubset = (CoverageCoordAxis1D) axiso.get();
    result.add(runAxisSubset);

    // subset on timeOffset (1a, 1c, 2a)
    if (params.hasTimeOffsetParam() || !params.hasTimeParam()) {
      axiso = timeOffset.subset(params);
      if (!axiso.isPresent())
        return Optional.empty(axiso.getErrorMessage());
      CoverageCoordAxis timeOffsetSubset = axiso.get();
      result.add(timeOffsetSubset);

      if (makeCFcompliant) // add a time cordinate
        result.add(makeCFTimeCoord(runAxisSubset, (CoverageCoordAxis1D) timeOffsetSubset)); // possible the twoD time case, if nruns > 1
      return Optional.of(result);
    }

    // subset on time, # runtimes = 1 (1b)
    if (runAxisSubset.getNcoords() == 1) {
      double val = runAxisSubset.getCoordMidpoint(0);   // not sure runAxis is needed. maybe use runtimeSubset
      CalendarDate runDate = runAxisSubset.makeDate(val);
      Optional<TimeOffsetAxis> too = timeOffset.subsetFromTime(params, runDate);
      if (!too.isPresent())
        return Optional.empty(too.getErrorMessage());
      TimeOffsetAxis timeOffsetSubset =  too.get();
      result.add(timeOffsetSubset);

      if (makeCFcompliant)
        result.add(makeCFTimeCoord(runAxisSubset, timeOffsetSubset));
      return Optional.of(result);
    }

    // tricky case 2b time (point only not range) = constant forecast dataset
    // data reader has to skip around the 2D times
    // 1) the runtimes may be subset by whats available
    // 2) timeOffset could become an aux coordinate
    // 3) time coordinate becomes a scalar,
    isConstantForcast.set(true);

    CalendarDate dateWanted;
    if (params.isTrue(SubsetParams.timePresent))
      dateWanted = CalendarDate.present();
    else
      dateWanted = (CalendarDate) params.get(SubsetParams.time);
    if (dateWanted == null)
      throw new IllegalStateException("Must have time parameter");

    double wantOffset = runAxisSubset.convert(dateWanted); // forecastDate offset from refdate
    double start = timeOffset.getStartValue();
    double end = timeOffset.getEndValue();
    CoordAxisHelper helper = new CoordAxisHelper(timeOffset);

    // brute force search LOOK specialize for regular ?
    List<Integer> runtimeIdx = new ArrayList<>();  // list of runtime indexes that have this forecast
    // List<Integer> offsetIdx = new ArrayList<>();  // list of offset indexes that have this forecast
    List<Double> offset = new ArrayList<>();      // corresponding offset from start of run
    for (int i=0; i<runAxisSubset.getNcoords(); i++) {
      // public double getOffsetInTimeUnits(CalendarDate convertFrom, CalendarDate convertTo);
      double runOffset = runAxisSubset.getCoordMidpoint(i);
      if (end + runOffset < wantOffset) continue;
      if (wantOffset < start + runOffset) break;
      int idx = helper.search(wantOffset - runOffset);
      if (idx >= 0) {
        runtimeIdx.add(i);  // the ith runtime
        // offsetIdx.add(idx);   // the idx time offset
        offset.add(wantOffset - runOffset);   // the offset from the runtime
      }
    }

    // here are the runtimes
    int ncoords = runtimeIdx.size();
    double[] runValues = new double[ncoords];
    double[] offsetValues = new double[ncoords];
    int count = 0;
    for (int k=0; k<ncoords; k++) {
      offsetValues[count] = offset.get(k);
      runValues[count++] = runAxisSubset.getCoordMidpoint(runtimeIdx.get(k));
    }

    CoverageCoordAxisBuilder runbuilder = new CoverageCoordAxisBuilder(runAxisSubset)
            .subset(null, CoverageCoordAxis.Spacing.irregularPoint, ncoords, runValues); // LOOK check for regular (in CovCoordAxis ?)
    CoverageCoordAxis1D runAxisSubset2 = new CoverageCoordAxis1D(runbuilder);

    CoverageCoordAxisBuilder timebuilder = new CoverageCoordAxisBuilder(timeOffset)
            .subset(runAxisSubset2.getName(), CoverageCoordAxis.Spacing.irregularPoint, ncoords, offsetValues); // aux coord (LOOK interval) ??
    CoverageCoordAxis1D timeOffsetSubset = new TimeOffsetAxis(timebuilder);

    CoverageCoordAxis scalarTimeCoord = makeScalarTimeCoord(wantOffset, runAxisSubset);

    // nothing needed for CF, the run coordinate acts as the CF time independent coord. timeOffset is aux, forecastTime is scalar
    return Optional.of(Lists.newArrayList(runAxisSubset2, timeOffsetSubset, scalarTimeCoord));
  }
public Optional<List<CoverageCoordAxis>> subset(SubsetParams params, AtomicBoolean isConstantForcast, boolean makeCFcompliant) {
    List<CoverageCoordAxis> result = new ArrayList<>();

    Optional<CoverageCoordAxis> axiso = runAxis.subset(params);
    if (!axiso.isPresent())
      return Optional.empty(axiso.getErrorMessage());
    CoverageCoordAxis1D runAxisSubset = (CoverageCoordAxis1D) axiso.get();
    result.add(runAxisSubset);

    // subset on timeOffset (1a, 1c, 2a)
    if (params.hasTimeOffsetParam() || !params.hasTimeParam()) {
      axiso = timeOffset.subset(params);
      if (!axiso.isPresent())
        return Optional.empty(axiso.getErrorMessage());
      CoverageCoordAxis timeOffsetSubset = axiso.get();
      result.add(timeOffsetSubset);

      if (makeCFcompliant) // add a time cordinate
        result.add(makeCFTimeCoord(runAxisSubset, (CoverageCoordAxis1D) timeOffsetSubset)); // possible the twoD time case, if nruns > 1
      return Optional.of(result);
    }

    // subset on time, # runtimes = 1 (1b)
    if (runAxisSubset.getNcoords() == 1) {
      double val = runAxisSubset.getCoordMidpoint(0);   // not sure runAxis is needed. maybe use runtimeSubset
      CalendarDate runDate = runAxisSubset.makeDate(val);
      Optional<TimeOffsetAxis> too = timeOffset.subsetFromTime(params, runDate);
      if (!too.isPresent())
        return Optional.empty(too.getErrorMessage());
      TimeOffsetAxis timeOffsetSubset =  too.get();
      result.add(timeOffsetSubset);

      if (makeCFcompliant)
        result.add(makeCFTimeCoord(runAxisSubset, timeOffsetSubset));
      return Optional.of(result);
    }

    // tricky case 2b time (point only not range) = constant forecast dataset
    // data reader has to skip around the 2D times
    // 1) the runtimes may be subset by whats available
    // 2) timeOffset could become an aux coordinate
    // 3) time coordinate becomes a scalar,
    isConstantForcast.set(true);

    CalendarDate dateWanted;
    if (params.isTrue(SubsetParams.timePresent))
      dateWanted = CalendarDate.present();
    else
      dateWanted = (CalendarDate) params.get(SubsetParams.time);
    if (dateWanted == null)
      throw new IllegalStateException("Must have time parameter");

    double wantOffset = runAxisSubset.convert(dateWanted); // forecastDate offset from refdate
    double start = timeOffset.getStartValue();
    double end = timeOffset.getEndValue();
    CoordAxisHelper helper = new CoordAxisHelper(timeOffset);

    // brute force search LOOK specialize for regular ?
    List<Integer> runtimeIdx = new ArrayList<>();  // list of runtime indexes that have this forecast
    // List<Integer> offsetIdx = new ArrayList<>();  // list of offset indexes that have this forecast
    List<Double> offset = new ArrayList<>();      // corresponding offset from start of run
    for (int i=0; i<runAxisSubset.getNcoords(); i++) {
      // public double getOffsetInTimeUnits(CalendarDate convertFrom, CalendarDate convertTo);
      double runOffset = runAxisSubset.getCoordMidpoint(i);
      if (end + runOffset < wantOffset) continue;
      if (wantOffset < start + runOffset) break;
      int idx = helper.search(wantOffset - runOffset);
      if (idx >= 0) {
        runtimeIdx.add(i);  // the ith runtime
        // offsetIdx.add(idx);   // the idx time offset
        offset.add(wantOffset - runOffset);   // the offset from the runtime
      }
    }

    // here are the runtimes
    int ncoords = runtimeIdx.size();
    double[] runValues = new double[ncoords];
    double[] offsetValues = new double[ncoords];
    int count = 0;
    for (int k=0; k<ncoords; k++) {
      offsetValues[count] = offset.get(k);
      runValues[count++] = runAxisSubset.getCoordMidpoint(runtimeIdx.get(k));
    }

    CoverageCoordAxisBuilder runbuilder = new CoverageCoordAxisBuilder(runAxisSubset)
            .subset(null, CoverageCoordAxis.Spacing.irregularPoint, ncoords, runValues); // LOOK check for regular (in CovCoordAxis ?)
    CoverageCoordAxis1D runAxisSubset2 = new CoverageCoordAxis1D(runbuilder);

    CoverageCoordAxisBuilder timebuilder = new CoverageCoordAxisBuilder(timeOffset)
            .subset(runAxisSubset2.getName(), CoverageCoordAxis.Spacing.irregularPoint, ncoords, offsetValues); // aux coord (LOOK interval) ??
    CoverageCoordAxis1D timeOffsetSubset = new TimeOffsetAxis(timebuilder);

    CoverageCoordAxis scalarTimeCoord = makeScalarTimeCoord(wantOffset, runAxisSubset);

    // nothing needed for CF, the run coordinate acts as the CF time independent coord. timeOffset is aux, forecastTime is scalar
    return Optional.of(Lists.newArrayList(runAxisSubset2, timeOffsetSubset, scalarTimeCoord));
  }
/*
1) single runtime
1a timeOffset
1b time or timeRange
1c none = constant runtime dataset
2) multiple runtimes
2a timeOffset       = constant offset dataset
2b time (not range) = constant forecast dataset