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**To**:*math-font-discuss@cogs.susx.ac.uk***Subject**:**Re: ligatures in math font.****From**:*alanje@cogs.susx.ac.uk (Alan Jeffrey)***Date**: Fri, 20 Aug 93 20:22 BST

>if anyone loads an >>MC-encoded font containing upright glyphs, in order to get the >>effect of ${\bf waffle + \Gamma}$. > >One of the major thing we decided is that MC IS NOT FOR MULTILETTER >identifiers. So this example is no good. Another one of the goals is to be compatible with current plain TeX. At the moment, the input $\bf waffle + fit$ is acceptable input for $\mathbf{waffle} + \mathbf{fit}$. The TeXbook (p. 164 in the 1991 printing) states that the input: $\bf a+b=\Phi_m$ will produce the `a', `b', `\Phi' and `m' in bold, and the `+' and `=' in non-bold. If we want to be upwardly compatible with this, we have to allow for an MC-encoded font containing bold upright Latin glyphs, *and* the five ligatures <fi>, <ff>, <fl>, <ffi> and <ffl>. If we don't allow these ligatures, then there are documents which currently work (such as `Let $\bf Aff$ be the category of Affine logics' or `${\bf if}\; x<0\; {\bf then}\; x:={-x}\; {\bf fi}$') which will break with the new encodings. (Yes, I know it would be `better' to input these as `Let $\mathbf{Aff}$...' or `$\mathbf{if}...' but we don't have the option of rewriting the TeXbook.) This argument also applies to \sf, \it, etc. For example, input such as `$\sf \Omega = fix\;I$' is very common in papers on the lambda-calculus. There is a conflict between two of our goals: upward compatibility is incompatible with insisting that MC has no text ligatures. So the question is, which of these is more important? IMHO, upward compatibility is much more important than ligaturing. I'd be very worried that if we produce an encoding where 1% of `kosher' plain TeX documents break, then we'll have a great deal of difficulty persuading users to switch to the new fonts. I don't particularly like including text ligatures in math fonts, but I think it is necessary. Alan.

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