\startuseMPgraphic{circled}
pickup pencircle scaled 1mm ;
drawoptions(withcolor (.6+uniformdeviate.35)*white) ;
fill fullcircle xscaled 1.5cm yscaled 1cm ;
drawoptions(withcolor .625yellow) ;
draw fullcircle xscaled 1.5cm yscaled 1cm ;
\stopuseMPgraphic
%\startnotmode[proof]
% \defineoverlay[circled][\useMPgraphic{circled}]
%\stopnotmode
\define[1]\Circled%
{\framed
[background=circled,frame=off,offset=overlay]
{\color[darkred]{#1}}}
\setuppagenumbering[location=]
\setupheadertexts[margin][][\hbox to \hsize{\hss\Circled\pagenumber\hss}]
\startuniqueMPgraphic{titled}
path p ; p := unitsquare xscaled \overlaywidth yscaled \overlayheight ;
pickup pencircle scaled 1mm ;
drawoptions(withcolor .625yellow) ;
draw llcorner p -- lrcorner p ;
setbounds currentpicture to p ;
\stopuniqueMPgraphic
\defineoverlay[titled][\uniqueMPgraphic{titled}]
\define[2]\ChapterCommand%
{\setbox0=\hbox to \leftmarginwidth{#1\hss}%
\hskip-\leftmarginwidth
\hskip-\leftmargindistance
\framed
[background=titled,frame=off,offset=0pt]
{\box0
\hskip\leftmargindistance
#2}}
\define[2]\TitleCommand%
{\framed
[background=titled,frame=off,offset=0pt]
{#2}}
\setuphead
[chapter]%,section,subsection]
[command=\ChapterCommand,
style=\ss\bf\tfd,
color=darkred]
\setuphead
[title]%,subject,sububject]
[command=\TitleCommand,
style=\ss\bf\tfd,
color=darkred]
\setupbodyfont[bonum,rm,12pt]
\showframe
\starttext
\startchapter[title={Introduction}]
This is the chapter for introduction.
\stopchapter
\startchapter[title={Function}]
Now, we study the function.
\stopchapter
\starttitle[title={References}]
\startitemize[n]
\item first
\item second
\item third
\stopitemize
\stoptitle
\stoptext