I think I intended the third option to include all comma-separated lists, including:
a, b, c = x # No brackets
(a, b, c) = x # Round brackets
[a, b, c] = x # Square brackets
a, = x # Single target with comma
Perhaps something like this would be clearer:
* If the target list is
* a comma-separated list of targets, with or without brackets, or
* a single target with a trailing comma, with or without brackets, or
* a single target in square brackets,
the object must be . . .
The empty target list case could also be merged. Then the only differentiation is between a single target (no comma, optional round brackets) and iterable unpacking.