What does \csappto does and where does it come from?

Question

What does the macro \csappto do, exactly? I've seen it in multiple code snippets throughout the website and on the internet in general, but I've never seen an explanation of what it does. I've looked up various packages' documentation to try and find it, but I've never been able to. Where is it defined? In the core of TeX? (It's not documented in tex.pdf either)

Answer 1

It is defined in the etoolbox package and can be used to patch macros. More precisely (from the etoolbox documentation):

• \appto<hook>{code}: This command appends arbitrarycode to a hook.
• \csappto{csname}{code}: Similar to \appto except that it takes a control sequence name as its first argument.