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4.2 Erroneous Usage

Because the TFM’s public API is so small, there are only two usage conditions, so we’ll start with those ones. Neither of these conditions is restartable at the level of TFM.

An attempt at accessing the top, middle, bottom, or repeated character of a non extensible character will signal a not-extensible usage error. Remember that you can check a character’s extensibility with the extensiblep predicate.

Usage Error: not-extensible


An attempt at retrieving a ligature or kerning for two characters from different fonts will signal a different-fonts usage error.

Usage Error: different-fonts


Finally, an attempt at changing the design size of a font to something that is not a real greater or equal to 1 will just signal a type error.