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[Texmacs-dev] Ligatures in "teletype" fonts

From: Miguel de Benito Delgado
Subject: [Texmacs-dev] Ligatures in "teletype" fonts
Date: Sun, 6 Jan 2013 22:50:29 +0100

Dear list,

  in a recent fix for bug #32893 ( 
https://savannah.gnu.org/bugs/index.php?32893 ), I added a check for tex fonts: 
whenever the family is teletype (i.e. "tt"), we should not run the ligatures 
and kerning program associated to the font via the .tfm file. This is done in 
Plugins/Metafont/tex_font.cpp and works just fine as a solution to the bug, but 
I wonder: why are there ligatures for teletype fonts in the first place? Also, 
given that the font metrics files are decades old I'm starting to fear that 
changing the ligatures might not be such a good idea after all.

Any ideas / comments?

For completeness sake: The command 

   tfmtopl ectt10.tfm

reveals the following ligatures:

   (LABEL O 140)
   (LIG O 140 O 20)
   (LABEL O 47)
   (LIG O 47 O 21)
   (LABEL O 55)
   (LIG O 55 O 25)
   (LIG O 177 O 177)
   (LABEL O 74)
   (LIG O 74 O 23)
   (LABEL O 76)
   (LIG O 76 O 24)
   (LABEL O 54)
   (LIG O 54 O 22)
   (LABEL O 41)
   (LIG O 140 O 275)
   (LABEL O 77)
   (LIG O 140 O 276)

Checking the T1 table in 
ftp://ftp.dante.de/pub/tex/macros/latex/doc/encguide.pdf one sees these are 
exactly those ligatures creating trouble for source code as reported in the bug 
tracker (commas, single quotes, dashes, etc.).

Deleting this section from the generated file and reconverting it with pltotf 
produces a font metrics file which seems to make more sense to me than the 
original one. However, as said before this tfm files are decades old so there 
has to be some reason why they are how they are.
Miguel de Benito.

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