class TUnfoldSys: public TUnfold

  TUnfoldSys : public TUnfold

  TUnfold is used to decompose a measurement y into several sources x
  given the measurement uncertainties and a matrix of migrations A

  TUnfoldSys adds error propagation of systematic errors to TUnfold
  Also, background sources (with errors) can be subtracted.


  * For most applications, it is better to use TUnfoldDensity *
  * instead of using TUnfoldSys or TUnfold                    *


  If you use this software, please consider the following citation
       S.Schmitt, JINST 7 (2012) T10003 [arXiv:1205.6201]

  More documentation and updates are available on
      http://www.desy.de/~sschmitt


  The following sources of systematic error are considered in TUnfoldSys

  (a) uncorrelated errors on the input matrix histA, taken as the
      errors provided with the histogram.
      These are typically statistical errors from finite Monte Carlo samples

  (b) correlated shifts of the input matrix histA. These shifts are taken
      as one-sigma effects when switchig on a given error soure.
      several such error sources may be defined

  (c) a systematic error on the regularisation parameter tau

  (d) uncorrelated errors on background sources, taken as the errors
      provided with the background histograms

  (e) scale errors on background sources

 In addition there is the (statistical) uncertainty of the input vector (i)

 Source (a) is providede with the original histogram histA
     TUnfoldSys(histA,...)

 Sources (b) are added by calls to
     AddSysError()

 The systematic uncertainty on tau (c) is set by
     SetTauError()

 Backgound sources causing errors of type (d) and (e) are added by
     SubtractBackground()


 NOTE:

    Systematic errors (a), (b), (c) are propagated to the result
       AFTER unfolding

    Background errors (d) and (e) are added to the data errors
       BEFORE unfolding

 For this reason:
  errors of type (d) and (e) are INCLUDED in the standard error matrix
  and other methods provided by the base class TUnfold:
      GetOutput()
      GetEmatrix()

  whereas errors of type (a), (b), (c) are NOT INCLUDED in the methods
  provided by the base class TUnfold.

 Accessing error matrices:

  The error sources (b),(c) and (e) propagate to shifts of the result.
  These shifts may be accessed as histograms using the methods
     GetDeltaSysSource()            corresponds to (b)
     GetDeltaSysTau()               corresponds to (c)
     GetDeltaSysBackgroundScale()   corresponds to (e)
  The error sources (a) and (d) originate from many uncorrelated errors,
  which in general are NOT uncorrelated on the result vector.
  Thus, there is no corresponding shift of the output vector, only error
  matrices are available

  Method to get error matrix       corresponds to error sources

   GetEmatrixSysUncorr()             (a)
   GetEmatrixSysSource()             (b)
   GetEmatrixSysTau()                (c)
   GetEmatrixSysBackgroundUncorr()   (d)
   GetEmatrixSysBackgroundScale()    (e)
   GetEmatrixInput()                 (i)
   GetEmatrix()                      (i)+(d)+(e)
   GetEmatrixTotal()                 (i)+(a)+(b)+(c)+(d)+(e)

  Error matrices can be added to existing histograms.
  This is useful to retreive the sum of several error matrices.
  If the last argument of the GetEmatrixXXX() methods is set to kFALSE,
  the histogram is not cleared, but the error matrix is simply added to the
  existing histogram