Under what circumstances is it allowed to connect:
Multiple execution output ports (those with the blank triangle) in different branches to one single execution input, effectively merging the branches into one.
Either an output port to an input in the same node, or to an input in a different node connected to the same branch but that is executed before. Both, for execution and data ports. Therefore effectively creating a feedback loop.
I know those rules are enforced by the editor, but I don't have one available at the moment.