Copying a proof almost verbatim I am writing a math paper which has an electronic supplementary document. In the supplement, I want to put a proof of a lemma. This proof is copied and pasted from my previous paper and I have changed some symbols (eg. H becomes X) to put it in context.  Is this allowed? I have written on it:

Proof (proof modified from Proof of Theorem 1 in [paper]).
Blah blah...

Is this plagiarism or not? It is only in the supplementary document (which is published online) and I have cited.
I agree this is not plagiarism if you make it clear that is an essentially verbatim extract from your previous paper.

However, it also seems unnecessary, especially if your previous paper is readily available online.  (And if it's not, why not?  Consider posting it to arXiv or your personal web page.)  In such a case, I would probably rather just refer the reader to the previous paper ("see Lemma 4.3 of [8]").  If you feel it's necessary, you can say something like "changing all occurrences of H to X", but I'd expect that a mathematical audience will just do that automatically without needing to think about it.