Here's how I added a new cut, Det_saturate:

I. First, define the new cut and its threshold.  (Threshold is
optional; it's a parameter used in checking for cut conditions.)

  1. In VaEvent.hh, add

    static Double_t fgSatCut;    // cut threshold from database

  This will be the threshold for the saturation cut, to be set from the
  database. 

  2. In VaEvent.hh, add 

    static Cut_t fgSatNo;        // cut number for saturation

  This will be the cut number corresponding to the saturation cut.

  3. In TaCutList.hh, add

    Cut_t fSatNo;                      // cut number for saturation

  This will be another copy of the cut number.

  4. In VaEvent.cc, add

    Double_t VaEvent::fgSatCut;

  and

    Cut_t VaEvent::fgSatNo;  

  to declare the static members.

  5. In VaEvent::RunInit(const TaRun& run), add

    fgSatCut = run.GetDataBase().GetCutValue("satcut");

  to set the cut threshold, and

    fgSatNo = run.GetDataBase().GetCutNumber ("Det_saturate");

  to set the value of the cut number.  Add to the conditional that
  checks for cut definitions:

    if (fgBurpNo == fgNCuts ||
	fgSatNo == fgNCuts ||
	fgLoBeamCNo == fgNCuts ||
	fgStartupNo == fgNCuts)
      {
	cerr << "VaEvent::RunInit WARNING: Following cut(s) are not defined "
	     << "in database and will not be imposed:";
	if (fgLoBeamCNo == fgNCuts) cerr << " Low_beam_c";
	if (fgBurpNo == fgNCuts) cerr << " Beam_burp";
	if (fgSatNo == fgNCuts) cerr << " Det_saturate";
	if (fgStartupNo == fgNCuts) cerr << " Startup";
	cerr << endl;
      }

  6. In TaCutList::Init(const TaDataBase& db) add

    fSatNo    = db.GetCutNumber (TaString ("Det_saturate"));

  7. In TaCutList::OKC (const VaEvent& ev) const exempt this cut from
     affecting Hall C OK:

	if (c->GetCut() != fLoBeamNo && 
	    c->GetCut() != fBurpNo &&
	    c->GetCut() != fSatNo &&
	    c->Inside(ev, fLowExtension[c->GetCut()], 
		      fHighExtension[c->GetCut()]) && 
	    c->GetVal() != 0)
	  oksofar = false;

  8. In TaDataBase::Print() print the cut threshold:

    clog << "   Saturation cut :  " << GetCutValue("satcut") << endl;

    and also in TaDataBase::Checkout():

    cout << "satcut  cut = " << GetCutValue("SATCUT") << endl;

  9. In TaDataBase::InitDB() add satcut to list of table names (append
     it to the end):

    tables.push_back("satcut");        //  37

      and set its type to numeric (d for double):

    if (i == 37) columns.push_back(new dtype("d"));  // satcut


II. Implement the check for cut conditions.  (This is optional.  You
can always just define the cut and add cut intervals manually to the
database, if you want.)

  1. In VaEvent.hh, add

    static UInt_t fgDet[4];      // index to detectors for cuts

     This was needed because this cut condition check was the first
  code in VaEvent to look at detector data values.  (N.B. later fgDet
  was changed to fgDetRaw, and 4 was changed to DETNUM.)

  1a. In VaEvent.cc, add

    UInt_t VaEvent::fgDet[4];

  to declare the static member.

  2. In VaEvent::RunInit(const TaRun& run), add

  Get the keys for the detectors:

    fgDet[0] = run.GetKey (string ("det1r"));
    fgDet[1] = run.GetKey (string ("det2r"));
    fgDet[2] = run.GetKey (string ("det3r"));
    fgDet[3] = run.GetKey (string ("det4r"));

     This was needed because this cut condition check was the first
  code in VaEvent to look at detector data values.

  3. In VaEvent::CheckEvent(TaRun& run), implement the check:

    if ( fgSatNo < fgNCuts)
      {
	Bool_t saturated = false;
	UInt_t i;
	for (i = 0; i < DETNUM && !saturated; i++) 
	  saturated = GetData (fgDet[i]) >= fgSatCut;

	if (saturated)
	  {
  #ifdef NOISY
	    clog << "Event " << fEvNum << " failed saturation cut, det"
		 << (i+1) << " raw = " << fData[key] << " > " << fgSatCut << endl;
  #endif
	    val = 1;
	  }
	AddCut (fgSatNo, val);
	run.UpdateCutList (fgSatNo, val, fEvNum);
      }


III.  For runs where we want this cut in effect, specify the cut and
its threshold in the database:

Add

    satcut 60000

and change

    ncuts 5
    cutnames Low_beam Beam_burp Evt_seq Pair_seq Startup
    extlo 10 10 25 25 0
    exthi 1200 40 25 25 1

to 

    ncuts 6
    cutnames Low_beam Beam_burp Evt_seq Pair_seq Startup Det_saturate
    extlo 10 10 25 25 0 25
    exthi 1200 40 25 25 1 25