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"FE I remains undetected"

Remains Undetected. Pretty crystal clear.

No. Undetected iron and zero iron are different. Source: astronomer.

"Undetected" is not an "upper bound." Source: computer scientist.

Undetected means you can compute an upper bound, based on the performance of your instrument. Again, these are undergraduate level concepts.

Again, you’re being obtuse but well leave it there. Good day.