Redefining Commands: Safety and Best Practices

There’s plenty of questions on this site about redefining specific commands, but I couldn’t find a more general, comprehensive question about the safety and best practices of redefining commands in LaTeX.

Answers to specific parts of this question are welcome, but most likely have already been formulated on this site somewhere; links to those would also be welcome, of course.

Specific recommendations for further reading (chapters in books, specific content on other websites, etc.) are also welcome.

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1. Redefining a Command: what are the implications for packages I’m loading that use said command, for lower-level places that use said command (e.g. the documentclass, or for higher-level places (mostly meaning just calls to commands from packages which directly or indirectly depended upon the old command), and how do they depend on:

   1. where the original command is defined?

      • defined in a package I’m loading, in the documentclass I’m using, in the TeX engine I’m using, my own preamble, etc.

   2. where the redefined command is defined?

      • before or after begin{document}?
      • before or after usepackage{ <package that depends on the old command> }?
      • in a small space in the document enclosed with bgroup and egroup that does / doesn’t make use of some / any functionality of the affected package(s).

   3. how the original command is defined?

      • newcommand, def, edef, protecteddef, NewDocumentCommand, DeclareExpandableDocumentCommand, DeclareRobustCommand, etc.
      • content of the original command (i.e. e.g. in the original newcommand{cmd}{ <old content> })

   4. how the redefined command is defined?

      • renewcommand, patchcmd, xpatchcmd, RenewDocumentCommand, RenewExpandableDocumentCommand, renewrobustcmd, etc.
      • content of the redefined command (i.e. e.g. renewcommand{cmd}{ <new content> })

   5. whether the old command is renamed with e.g. letoldcmd=cmd

      • how it is renamed?

   6. how the old command was being made use of?

2. Redefining a TeX Primitive (e.g. neq or par)…

   1. Is it always a no-go, or does it depend on the specific primitive or "type" thereof?
   2. Same questions as above

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What led me to ask this question, which has popped into my head plenty of times before, was my intention to redefine the TeX primitive d, whose original definition is (TeXbook p. 356):

defd#1{oalign{#1crcrhidewidth.hidewidth}}

I wanted to redefine it into an all-purpose derivative operator, that e.g. becomes frac{mathop{}!mathrm{d}f}{mathop{}!mathrm{d}x} when given two arguments, mathop{}!mathrm{d} when given one, uses partial when starred, higher derivatives if given an optional argument, etc. How to achieve this particular behavior is, of course, not this question.

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Related resources

• Overleaf has a handy list of TeX primitives
• Display source for a command
• Documentation reference for LaTeX internal commands?
• Definitive way to pre- and append code, or patch commands?
• Short names for macros