Since this is a logical error, no logical errors are "obvious" a priori when one tries to debug any code except maybe for the 10-line-long ones. One's instinct or debugger does not always readily point him to qr// as one of the potential sources of errors.

In reply to Re: Risks in the oblivious use of qr// by chunlou
in thread Risks in the oblivious use of qr// by gmax

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