Leave a comment in a formula

Generic formula
=formula+N("comment")
Summary
Explanation
This is a tricky use of N() that allows you to use it as a way to leave in-cell comments. It only works for formulas that return numeric results.
The N function takes a value and returns a number. When given a text value, the N function returns zero.
In this case, the primary formula the SUM function, and the N function simply evaluates the comment text and returns zero:
The comment is visible in the formula bar when the cell is selected.
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