% posted on list (adapted)

\setupinteraction[state=start]

% \startsectionblockenvironment[bodypart]
% 	\setcounter[userpage][9]
% \stopsectionblockenvironment

\setupuserpagenumber[way=byblock]

\starttext

\startfrontmatter
    dummy 1\page
    dummy 2\page
\stopfrontmatter

\startbodymatter
    dummy 3\page
    bla\index{bla}
\stopbodymatter

\placeregister[index]

\stoptext
