TeX accepts S'^{2} (between $ delimiters) as a valid construction
(TeXbook, p.130, line above Ex. 16.5) but mathed in LyX converts this to
S'{}^{2}. In this simple case that is not a problem (in fact the
interpolated {} gives better spacing) but it becomes a problem with
expressions like (real-life example)
S'^{\,-1}_{0i} or S_{0i}'^{\,-1}
These compile fine in TeX and display as desired but mathed turns them into
S'{}_{0i}^{\,-1} and S_{0i}'{}^{\,-1}
which give very different and clearly wrong results in the pdf.
The combination of a prime followed by a superscript seems a
sufficiently basic sequence to me that I think mathed should handle it
with more finesse than immediately and always inserting an empty brace
pair between ' and ^.
[[Speculation: I don't know the code but perhaps the problem is that
mathed insists on subscript preceding superscript when both are involved
in a symbol? R^{ij}_{kl} in mathed becomes R_{kl}^{ij}, and inserting a
prime as in R'_{kl}^{ij} then produces the double superscript error. In
contrast, R_{kl}'^{ij} does not. TeX accepts the '^ combination and I
think mathed should do so too.]]
Andrew
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