As underlining or bolding are very specific operations depending strongly on the OS, why not use good old simple textual measures to get the readers attention, e.g. 

Please supply only *one* argument.
Please supply only _one_ argument.
Please supply only one(!) argument.

[download]
This works everywhere where you can print text :)

In reply to Re: How do I print characters with underline or bold? by Hofmator
in thread How do I print characters with underline or bold? by eyal_kle

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