How come this doesn't work ?

\defineblock[prev]
\defineblock[full]

\hideblocks[full]
%\hideblocks[prev]

\def\previewMsg{This part is hidden}
\def\showPreviewMsg{\beginprev \hairline \previewMsg \hairline \endprev}

\starttext

\input tufte

\showPreviewMsg      % it doesn't show
%\beginprev
%    \hairline
%    \previewMsg
%    \hairline
%\endprev
\beginfull
    \input knuth
\endfull

\input tufte

\stoptext

Alan

P.S. What is the \processblocks command for ?