[Tagging] Route members: ordered or not

Volker Schmidt voschix at gmail.com
Thu May 3 16:42:38 UTC 2018

I will try to explain this in a more systematic way:

Routes belong to either of two categories:
(A) Those whose members can be sorted into a single ordered sequence
(B) Those that cannot be sorted into a single ordered sequence of members
Sorting makes only sense for category (A)
Routes of type (B) can be subdivided into routes of type (A), each of which
can be sorted, but the overall route can not be sorted.

Routes are of type (A) if
(1) the path from begin to end is identical to the reverse path with  all
members traversed in the reverse order and in the opposite direction
(2) all members have the role forward
(3) all members have the role backward

Any route
(1) that has more than two ends
(2) that contains any loop (except the case that the entire route is a
single loop)
(3) that contains any element with role forword or role backward (except
the cases of all-forward or all-backward)
(4) that contains node or area elements
is of type B

I am not sure if I have taken care of all cases - please complete as
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