class THaCut: public THaFormula

 Create a cut 'name' according to 'expression'.
 The cut may use global variables from the list 'vlst' and other,
 previously defined cuts from 'clst'.

 Unlike the behavior of THaFormula, THaCuts do NOT store themselves in
 ROOT's list of formulas. Otherwise existing cuts used in new cut expressions
 will get reparsed instead of queried. As a result, existing cuts can only
 be used by other cuts, not by formulas.

 Copy ctor

 Check if 'name' is in the list of existing cuts.
 If so, store pointer to the cut for use by DefinedValue().
 Otherwise, assume 'name' is a global variable and pass it on
 to THaFormula::DefinedVariable().

 Print this cut.
 Options:
   "BRIEF" -- short (default)
   "FULL"  -- extended
   "LINE"  -- one-line (for listings)
   "STATS" -- one-line (listing of statistics only)

 Set name of block (=group of cuts) for this cut.
 The block name is used only for informational purposes within the
 THaCut class.

 Set name of the cut
 This method overrides the one provided by TNamed
 so that the name can only be set if it is empty.
 This avoids problems with hash values since THaCuts are
 elements of a THashList.
 Alternatively, we could rehash the associated table(s), but that
 would be much more involved.

 Set name and formula of the object, only if not yet initialized

 Evaluate the cut and increment counters

 Reset cut result and statistics counters