Inconsistency regarding possibility of trust chain loops

[zachmann]

Section 10.1 says

Federation participants MUST NOT attempt to fetch Entity Statements they already have obtained during this process to prevent loops. An invalid_trust_chain error SHOULD be returned if a loop is encountered.

Section 16.1 Federation topologies states

Also, as described in Section 10.1 Federation topologies MUST NOT contain loops.

However, I cannot see that 10.1 forbids loops in the federation topology. It only states that participants MUST NOT fetch stmts they already have, i.e. this is a mechanism to deal with the possibility of loops.

Do we want to forbid trust chain loops? Then this should be made more prominent in the spec and 10.1 should be adapted. If not, the statement should be removed from 16.1

[cicnavi]

This is from #161, addressed with PR #164