This is needed since that operator returns a local copy, not
an lvalue. Attempting to write to the return value of these operators
wouldn't change the array element. PoolVectors need locking when
writing, so this operator can't return a writable reference.
To update a Pool*Array, use the `set()` method which locks and unlocks
the array. For multiple writes, use the `write()` method which returns
a locked writable view, and unlocks when it goes out of scope.
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