The output list makes the tuples different than the input... there is no way that john suddenly owes beryl 25 for example. I don't see an explanation for this

The output list makes the tuples different than the input... there is no way that john suddenly owes beryl 25 for example. I don't see an explanation for this in the text - but a directed edge seems to switch from a semantic of "owes" to "is owed".