Binning scheme:
 =================================
TUnfoldBinning "binning" has 20 bins [1,21] nTH1x=20
  TUnfoldBinning "branch1" has 12 bins [1,13] nTH1x=12
   distribution: 12 bins
    "x" nbin=4
    "y" nbin=3
  TUnfoldBinning "branch2" has 6 bins [13,19] nTH1x=6
   distribution: 6 bins
    "x" nbin=3
    "y" nbin=2
  TUnfoldBinning "branch3" has 2 bins [19,21] nTH1x=2
   distribution: 2 bins
    "x" nbin=2

CreateEmptyBinMap
=======================
global bin:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21
mapped to:  -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1

branch1->FillBinMap1D(...,"y[C]",...,2)
=======================
global bin:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21
mapped to:  -1  2  3  4  5  2  3  4  5  2  3  4  5 -1 -1 -1 -1 -1 -1 -1 -1 -1

dest |contributing bins
=====+======================================
   2 |#1 (branch1:y[0,1]:x[0,1])
   2 |#5 (branch1:y[1,2]:x[0,1])
   2 |#9 (branch1:y[2,3]:x[0,1])
=====+======================================
   3 |#2 (branch1:y[0,1]:x[1,2])
   3 |#6 (branch1:y[1,2]:x[1,2])
   3 |#10 (branch1:y[2,3]:x[1,2])
=====+======================================
   4 |#3 (branch1:y[0,1]:x[2,3])
   4 |#7 (branch1:y[1,2]:x[2,3])
   4 |#11 (branch1:y[2,3]:x[2,3])
=====+======================================
   5 |#4 (branch1:y[0,1]:x[3,4])
   5 |#8 (branch1:y[1,2]:x[3,4])
   5 |#12 (branch1:y[2,3]:x[3,4])
=====+======================================

branch2->FillBinMap1D(...,"x[C]",...,7)
=======================
global bin:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21
mapped to:  -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1  7  7  7  8  8  8 -1 -1 -1

dest |contributing bins
=====+======================================
   7 |#13 (branch2:y[0,1]:x[0,1])
   7 |#14 (branch2:y[0,1]:x[1,2])
   7 |#15 (branch2:y[0,1]:x[2,3])
=====+======================================
   8 |#16 (branch2:y[1,2]:x[0,1])
   8 |#17 (branch2:y[1,2]:x[1,2])
   8 |#18 (branch2:y[1,2]:x[2,3])
=====+======================================

binning->FillBinMap1D(...,"y[C]",...,1)
=======================
global bin:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21
mapped to:  -1  1  2  3  4  1  2  3  4  1  2  3  4  5  6  7  5  6  7  8  9 -1

dest |contributing bins
=====+======================================
   1 |#1 (branch1:y[0,1]:x[0,1])
   1 |#5 (branch1:y[1,2]:x[0,1])
   1 |#9 (branch1:y[2,3]:x[0,1])
=====+======================================
   2 |#2 (branch1:y[0,1]:x[1,2])
   2 |#6 (branch1:y[1,2]:x[1,2])
   2 |#10 (branch1:y[2,3]:x[1,2])
=====+======================================
   3 |#3 (branch1:y[0,1]:x[2,3])
   3 |#7 (branch1:y[1,2]:x[2,3])
   3 |#11 (branch1:y[2,3]:x[2,3])
=====+======================================
   4 |#4 (branch1:y[0,1]:x[3,4])
   4 |#8 (branch1:y[1,2]:x[3,4])
   4 |#12 (branch1:y[2,3]:x[3,4])
=====+======================================
   5 |#13 (branch2:y[0,1]:x[0,1])
   5 |#16 (branch2:y[1,2]:x[0,1])
=====+======================================
   6 |#14 (branch2:y[0,1]:x[1,2])
   6 |#17 (branch2:y[1,2]:x[1,2])
=====+======================================
   7 |#15 (branch2:y[0,1]:x[2,3])
   7 |#18 (branch2:y[1,2]:x[2,3])
=====+======================================
   8 |#19 (branch3:x[0,1])
=====+======================================
   9 |#20 (branch3:x[1,2])
=====+======================================