P.S.

For your conveniance, the minimal example:

\startMPpositiongraphic{mypos:arrow}
   pair c[] ;
   initialize_box(\MPpos{\MPvar{self}});
   c[1] := cxy ;
   initialize_box(\MPpos{\MPvar{to}});
   c[2] := cxy ;
   drawarrow c[1] -- c[2] withpen pencircle scaled 2bp ;
\stopMPpositiongraphic
\startpositionoverlay{arrow}
\setMPpositiongraphic{X-1}{mypos:arrow}{to=X-2}
\setMPpositiongraphic{X-2}{mypos:arrow}{to=X-3}
\stoppositionoverlay
\defineoverlay[arrow][\positionoverlay{arrow}]
\setupbackgrounds[page][background=arrow]


\starttext

\section {Anchors and layers}

In a previous section we saw that some \hpos{X-1}{words} were
\hpos{X-2}{circled} and connected by an \hpos{X-3}{arrow}.
As with most things in \CONTEXT, marking these words is separated
from declaring what to do with those words. This paragraph is keyed
in as:

\stoptext