calculation.py:traverse() test failure

[drf5n]

I get an error when running the self-test on calculations.py:

$ python calculation.py
Result for impossible intersection:
p4 -3.82805968326e+17 -3.82805968326e+17
Result for observe each other:
p5 181.019335984 181.019335984
Traceback (most recent call last):
  File "calculation.py", line 1046, in <module>
    [s114otra,o114_113otra,o114_115otra],[s115otra,None,None] ] )
  File "calculation.py", line 407, in traverse
    pcode = (trav_obs[i][0].p.pc if trav_obs[i][0].p.pc is not None else "-")
AttributeError: 'NoneType' object has no attribute 'pc'

it is this line:

https://github.com/zsiki/ls/blob/master/calculation.py#L407

triggered by the call around here:

https://github.com/zsiki/ls/blob/master/calculation.py#L1046

I'm not sure if the input to the test is valid or invalid, or if traverse() should protect against that final point.