what is nxy?

["Denis B. Roegel" <Denis.Roegel@loria.fr>]

If I go back to the example of an earlier question of mine,
I see that

  1) I don't understand what `nxy' is; as far as I can see,
     it is not explained in the metafun manual;
  2) I don't understand why the connection between the first
     and the second word goes from center to center, 
     and this may have something to do with 1).

Thanks!

Denis

\starttext

\startMPpositiongraphic{mypos:circle}
  initialize_box(\MPpos{\MPvar{self}});
  path p; p:=llxy..lrxy..urxy..ulxy..cycle;
  pickup pencircle scaled 1pt;
  fill p withcolor .800white;
  draw p withcolor .625yellow;
  anchor_box(\MPanchor{\MPvar{self}});
\stopMPpositiongraphic

\startMPpositiongraphic{mypos:line}
  path pa, pb, pab ; numeric na, nb;
  initialize_box(\MPpos{\MPvar{from}});
  na := nxy ; pa := llxy..lrxy..urxy..ulxy..cycle ;
  initialize_box(\MPpos{\MPvar{to}});
  nb := nxy ; pb := llxy..lrxy..urxy..ulxy..cycle ;
  if na=nb :
    pab := center pa -- center pb ;
    pab := pab cutbefore (pab intersectionpoint pa) ;
    pab := pab cutafter (pab intersectionpoint pb) ;
    pickup pencircle scaled 1pt;
    drawarrow pab withcolor .625yellow;
    anchor_box(\MPanchor{\MPvar{from}});
  fi;
\stopMPpositiongraphic

\startpositionoverlay{backgraphics} 
\setMPpositiongraphic{X-1}{mypos:circle}
\setMPpositiongraphic{X-2}{mypos:circle}
\setMPpositiongraphic{X-3}{mypos:circle}
\stoppositionoverlay
\startpositionoverlay{foregraphics}
\setMPpositiongraphic{X-1}{mypos:line}{to=X-2}
\setMPpositiongraphic{X-2}{mypos:line}{to=X-3}
\stoppositionoverlay

\defineoverlay [foregraphics] [\positionoverlay{foregraphics}]
\defineoverlay [backgraphics] [\positionoverlay{backgraphics}]

\setupbackgrounds
    [page]
    [background={backgraphics,foreground,foregraphics}]

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. 

\stoptext