validation: erroneous failure?

[oliver-sanders]

This fails here and on master, but I wonder if it needs to?

@x & (a | b) => c

$ cylc val .
WorkflowConfigError: Xtriggers cannot be used in conditional graph expressions:
@x&(a:succeeded|b:succeeded) => c

It's been a long time since I looked at - and probably wrote - the associated code, so I'm not sure of the reason for the restriction anymore, and whether it should apply to this sort of expression (and I'm out of time today). But possibly it should not fail because it's equivalent to this:

@x => c
a | b => c

-- #6772 (comment)