Cfrgtkky

The following code shows how amscd and mtpro2 do not play well together with the subscriptcorrection option enabled:

documentclass{article}



usepackage[subscriptcorrection]{mtpro2}

usepackage{amscd}



begin{document}



[

 begin{CD}

     H_{R}(U) @>d >> B

 end{CD}

]



end{document}

This halts with the error

! Package amscd Error: Invalid use of @.

It can be sort of corrected with

 disablesubscriptcorrection

before the diagram, but the results are less than optimal.

I expect that the problem has to do with catcode values of _ like in here, but nonetheless the conflict is there. The question is:

Who is at fault here? amscd or mtpro2? Or even LaTeX for now allowing interaction between packages.

The problem clearly is in the subscript: mtpro2 makes _ math active and its expansion throws in a redefinition of the scratch macro next, which CD expects not to change.

Whose fault is it? This question has no answer, I'm afraid. Possibly the most culpable is CD, where next is used in a dubious way, assuming its meaning will not change for a long span of time. However, it's quite late for changing it.

A simple workaround would be protecting the entry by braces.

begin{CD}

  {H_{R}(U)} @>d >> B

end{CD}

The issue can also be fixed on the mtpro2 side using a different command name than next in a crucial macro.

documentclass{article}

usepackage{amsmath}

usepackage{amscd}

usepackage[subscriptcorrection,lite]{mtpro2}



makeatletter

begingrouplccode`~=`_

lowercase{endgroupdef~}{futureletnext@@@s@@b}

defs@@b{ifcatrelaxnoexpandnext@@@expandaftersbelse

 expandafters@@b@fi}

makeatother



begin{document}



[

 begin{CD}

     H_{A}(U) @>d >> B

 end{CD}

]

[

disablesubscriptcorrection

 begin{CD}

     H_{A}(U) @>d >> B

 end{CD}

]



end{document}

I added the version with disablesubscriptcorrection to show that the correction is indeed done as expected in the first diagram.

There are much better packages for diagrams, which won't suffer from this problem.

Just by way of example, here's a diagram impossible to draw with CD; it sports the subscript correction and also arrow matching those of mtpro2.

documentclass{article}

usepackage{amsmath}

usepackage[subscriptcorrection,lite]{mtpro2}

usepackage{tikz-cd}



tikzcdset{arrow style=math font}



begin{document}



[

begin{tikzcd}

H_{A}(U) arrow[r,"d"] arrow[dr,"varphi"] &

B arrow[d]

\

& C

end{tikzcd}

]



end{document}