I have a strange recollection playing with Linux text processing command line tools in the 90s and stumbling upon something that looks like the "concordance" ou

I have a strange recollection playing with Linux text processing command line tools in the 90s and stumbling upon something that looks like the "concordance" output and having no idea why it might be useful, nor any way to find out. Now I know! (And I can finally fix a dangling reference in my brain.)