can include "{lineno}"
to have `lineno` interpolated into it.

As an example, this code::

    if something(x):        # line 1
        func(x)             # line 2
    more_stuff()            # line 3

would have two ArcStarts:

- ArcStart(1, "the condition on line 1 was always true")
- ArcStart(1, "the condition on line 1 was never true")

The first would be used to create an arc from 1 to 3, creating a message like
"line 1 didn't jump to line 3 because the condition on line 1 was always true."

The second would be used for the arc from 1 to 2, creating a message like
"line 1 didn't jump to line 2 because the condition on line 1 was never true."

r