Initial Vec A
Vec Object: 1 MPI process
0.
1.
2.

Broadcast A->B over sfA
Vec Object: 1 MPI process
2.
0.
1.

Broadcast B->BA over sfB
Vec Object: 1 MPI process
10.
2.
0.

Broadcast A->BA over sfBA (sfB o sfA)
Vec Object: 1 MPI process
11.
2.
0.
PetscSF Object: sfA 1 MPI process
  [0] Number of roots=3, leaves=3, remote ranks=1
  [0] 0 <- (0,2)
  [0] 1 <- (0,0)
  [0] 2 <- (0,1)
PetscSF Object: sfB 1 MPI process
  [0] Number of roots=3, leaves=2, remote ranks=1
  [0] 1 <- (0,0)
  [0] 2 <- (0,1)
PetscSF Object: (sfB o sfA) 1 MPI process
  [0] Number of roots=3, leaves=2, remote ranks=1
  [0] 1 <- (0,2)
  [0] 2 <- (0,0)