@JosephWright "If \pdfoutput isn't defined then \ifnum\relax>0 crashes.
@wipet Of course, but the number test here is only expanded if the earlier check for existence of \pdfoutput passes" OK, but if `\pdfoutput` is defined (checked by previous pass) then you needn't to put it between `\csname...\endcsname`. Uff.