[Q] nonexistent characters

[Didier Verna]

  Hello,

I'm a bit confused about the notion of nonexistent characters in a font
loaded from a TFM file. There are two situations in which a character
doesn't exist: if its code is not within [bc,ec], or if it is, but its
metrics are all 0.

In the TFM description, it is explicitly said that a right boundary
character's code may not lie between bc and ec, which I think I
understand, because this character is in fact never typeset.

However, I have found fonts in which some nonexistent characters exist
within [bc,ec], notably with an index to the lig/kern table. I fail to
understand the purpose of these characters. Why would a font have a
nonexistent character, yet part of a ligature or kerning operation? Is
this related to accentuating previous characters, or anything else?

Thanks!

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