http://catb.org/~esr/jargon/html/M/metasyntactic-variable.html https://pageperso.lif.univ-mrs.fr/~edouard.thiel/divers/variables-metasyntaxiques.html
foo, bar, baz, qux, quux, corge, grault, garply, waldo, fred, plugh, xyzzy, thud (rfc 3092) bat qwe zxc asd