Internally a PBR stores its adjacencies on [0, ..., 2n -1] but they appear to be defined on [-n, ..., -1, 1, ..., n]. At present the adjacencies expose the former, but should probably expose the latter.
In [3]: x = PBR([[-1, 1], [2]], [[], []])
In [4]: x[1]
Out[4]: [1]
In [5]: x[2]
Out[5]: []
In [6]: x[0]
Out[6]: [0, 2]
Internally a
PBRstores its adjacencies on[0, ..., 2n -1]but they appear to be defined on[-n, ..., -1, 1, ..., n]. At present the adjacencies expose the former, but should probably expose the latter.