[NTG-context] context-2011-mathml-update.pdf FlushStep and mupdf

Hi, As suggested a few months ago in thread "which command would give the same effect as \pause in beamer ?", I am digging in context example to use the step mecanism for presentation. I grepped the example folder and founded files using \FlushStep, for example : context-2011-mathml-update.tex The trouble is that when opening context-2011-mathml-update.pdf with mupdf I do not see anything similar to beamer \pause. The whole slides are shown in one piece. I tried several other pdf with Steps mecanism (context-2011-ebook-export, context-2012-thescript, context/2013/context-2013-speed.pdf). Steps weren't displayed in any case. Did I miss something or is it a viewer issue? Thanks, Damien