Historically, a 'when' condition performed an AST-traversal to locate
any index-expressions like `x[9]` and evaluated them so that it could
register the associated value as something for which it needs to receive
"modification" notifications.
Evaluating arbitrary expressions during an AST-traversal like that ignores
the typical order-of-evaluation/short-circuiting you'd expect if the
condition was evaluated normally, from its root expression.
Now, a new subclass of IndexExpr is used to keep track of all IndexExpr
results in the context of evaluating a 'when' condition without having
to do a secondary AST-traversal-and-eval. i.e. the first evaluation of
the full 'when' condition follows the typical expression-evaluation
semantics (as always), but additionally now captures all the values
a Trigger needs to monitor for modifications.
