random_expression: issues with tuple sets
- we can still generate '\e|\e' as a label, even if we did not ask for \e.
- this should be also checked for lan
- when we generate for lat, we generate tuple operators even if we did not ask for it. The reason is that we generate multitape labels, and pretty-print them, which turns them into multitape expressions. And with identities = none, that's all we have left: tupling operator on single-tape labels.