Turning on -Wformat-noliteral for Mac triggered a new warning.
Blindly suppressing the warning would have silenced it but would
also have left a real bug in place. The former format was passing
a string argument to %d format.
This converts the format to a literal with an additional argument
for the non-literal part. It compiles cleanly but I don't know how
to test it, let alone force an error for it to report.
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