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This allows subscript access into model identifiers, both direct (e.g.,
"my_identifier[0]" as well as indirect, e.g., "my_identifier[index]",
where |index| is a model identifier that points to a single integer.
This can be nested arbitrarily deeply, i.e., "a[b[c[d[...]]]]".

Note that subscript access into UTF-8 model identifiers is NOT supported.
Specifically, identifiers must match a regular expression \w+ for
subscript support.

Bug: b/145043394
Change-Id: I03ca5c2d2c11a8c9c014a8cb3ad1768b7c13a62e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2139725
Commit-Queue: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Sandro Maggi <sandromaggi@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#758784}