Semantics of ZeroOrOnePath of property paths.

Hello,

I have a question about the semantics of ZeroOrOnePath of property paths.
In [1] ZeroOrOnePath with two variables is defined as:

eval(Path(X:var, ZeroOrOnePath(P), Y:var)) =
     { (X, xn) (Y, yn) | either (yn in nodes(G) and xn = yn) or {(X,xn), (Y,yn)} in eval(Path(X,P,Y)) }

Let's consider the following example graph G consisting of the following two triples:
<foaf:alice foaf:knows foaf:bob>
<foaf:bob foaf:knows foaf:bob>

Now we want to answer the following query:
SELECT * WHERE {?s <foaf:knows>? ?o}

Based on the former definition, this query would produce the following results:
1) {(?s, foaf:alice), (?o, foaf:alice)}, since foaf:alice is in nodes(G)
2) {(?s, foaf:alice), (?o, foaf:bob)}, since <foaf:alice foaf:knows foaf:bob> is in G
But {(?s, foaf:bob), (?o, foaf:bob)} is*no*  result, since foaf:bob is in nodes(G)*and*  <foaf:bob foaf:knows foaf:bob> is in G. This means the XOR condition in the definition of ZeroOrOnePath will be evaluate to false.

Based on my intuition of the semantics of ZeroOrOnePath I would have expected that {(?s, foaf:bob), (?o, foaf:bob)} is part of the result. Also in papers like [2] ZeroOrOnePath are defined in a way so that {(?s, foaf:bob), (?o, foaf:bob)} would be part of the result.

I would assume that there is a reason why the definition in [1] excludes {(?s, foaf:bob), (?o, foaf:bob)} from the result set. Could someone please explain me the reason, why {(?s, foaf:bob), (?o, foaf:bob)} was excluded?

Thank you very much for your help.

Best regards,
Adrian

[1]https://www.w3.org/TR/sparql11-query/#PropertyPathPatterns
[2]https://users.dcc.uchile.cl/~mromero/papers/ppath.pdf