Add cycle validation helpers and tighten cycle prevention

[evolve75]

Add explicit cycle validation helpers and harden add-cycle prevention to avoid infinite loops.

[RafaelTurtle]

Hello, I would like to help you solve this bug. Can I take this issue?

[evolve75]

Hello @RafaelTurtle , thank you! I have been working on this for the next release (see commits 83b82aa and fac77fb on the next branch. Note that the branch includes additional fixes and enhancements.

Have a look and let me know!

[evolve75]

next already contains the cycle validation and prevention work in 83b82aa .
Keeping the issue open for tracking, but the requested cycle-hardening is already present there.