`TriggerExpressionError` with inter-cycle dependency and OR operator

[MetRonnie]

Reproducible Example

[scheduler]
    allow implicit tasks = True
[scheduling]
    cycling mode = integer
    [[graph]]
        P1 = """
            a & x
            x[-P2] | a => b
        """

$ cylc play workflow -N
...
ERROR - Workflow shutting down - TriggerExpressionError: "-1/x succeeded|1/a succeeded":
    invalid syntax (<string>, line 1)
ERROR - No platforms available to remote tidy install targets:
     * localhost

[hjoliver]

It's OK with x[-P1] | a => b or with x[-P2] => b.

Validation gives a slight different error, but only because it catches TriggerExpressionError and prints a custom message.

[hjoliver]

We haven't replaced -1/x succeeded with Python code in the conditional expression for prerequisite evaluation.