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**To**:*math-font-discuss@cogs.susx.ac.uk***Subject**:**More on subscripts and superscripts****From**:*alanje@cogs.susx.ac.uk (Alan Jeffrey)***Date**: Mon, 9 Aug 93 15:39 BST

As far as arrow control sequences in sub- and super-scripts goes, I think we have three options: 1) Allow all of the CM* and MS*M arrows to be accessed directly by \mathchardef's. 2) Allow the CM* arrows to be accessed directly by \mathchardef's, but allow some of the MS*M glyphs to be accessed indirectly (e.g. by ligaturing and kerning). 3) Allow none of the arrows to be accessed directly by \mathchardef's. Advantages of each approach: 1) All (La)TeX and AmS(La)TeX documents will be compatible. 2) All (La)TeX documents will be compatible, although AmS(La)TeX documents which include input such as `$X_\nrightarrow$' will break. Most of the arrow glyphs will be accessed indirectly via the arrow-building kit, which cuts down on the number of glyphs required. All of the directly-accessed glyphs are very common, so should be available in every math font. 3) The arrow-building kit will be simpler and smaller, and won't have nasty kluges to allow for directly accessed glyphs. My preference is to go for option (2), as a reasonable compromise between compatibility and cleanness. This means that documents which contain input like `$X_\nrightarrow$' will break. The TeXbook states that a subscript is of the form: <subscript><math field> where: <math field> ::= <math symbol> | <filler> {<math mode material>} <math symbol> ::= \mathchar<15-bit number> | <mathchardef token> | \delimiter<27-bit number> So it's kosher plain TeX to say $X_\foo$ when \foo is a <mathchardef token>. So one of the requirements for upward compatibility with plain TeX is that any control sequence that is currently a <mathchardef token> must still be a <mathchardef token> in any upgrade. Is there a similar definition for AmSTeX? Is it kosher AmSTeX to give the input `$X_\nrightarrow$'? Should we be prepared to let the odd document containing `$X_\nrightarrow$' and the like influence the arrow-building kit? How upwardly compatible should we aim to be? These questions and more will not be answered on this week's installment of... Soap. Alan.

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