Metapost error related with ConTeXt

Question

I try to reuse this code from fpast (Generating Random arrows in tikz) in context but I found a error message associated with the freelabel() function. If I try to add the labels with label() function or by "draw textext()" method I found the same error. I think that this error is related to context side.

! Declared variable conflicts with previous vardef.

               ; <*> ...idate, random_vector[], bb_candidate, bb[];
                                               vardef bb(expr p) = save ...

! Inconsistent equation (off by -20). ; <*> ...enddef; pickup pencircle scaled 2bp; i = 1; forever: exitunless (i <=...

\starttext

\startMPcode
numeric u, nb_vectors, wd;
u := 1cm; % unit length
nb_vectors := 20; wd = .5; 
boolean intersect; pair random_origin; color random_color; path candidate, random_vector[], bb_candidate, bb[];
%
vardef bb(expr p) =
  save orig, extr; pair orig, extr; orig = point 0 of p; extr = point 1 of p;
  pair v,w; v = wd*unitvector(extr-orig)*u; w = v rotated 90;
  (orig - v + w) -- (extr + v + w) -- (extr + v - w) -- (orig - v - w) -- cycle
enddef;
%

  pickup pencircle scaled 2bp;
  i = 1; 
  forever: 
    exitunless (i <=  nb_vectors);
    intersect := false;
    random_length := u*(2+uniformdeviate(1));
    random_angle := uniformdeviate(359);
    random_origin := u*(uniformdeviate(wd*nb_vectors), uniformdeviate(wd*nb_vectors));
    random_color := (uniformdeviate(1), uniformdeviate(1), uniformdeviate(1));
    candidate := random_origin -- (random_origin + random_length*dir(random_angle));
    j := 1;
    forever:
      exitif (j = i) or (intersect = true);
      if bb(random_vector[j]) intersectiontimes bb(candidate) <>  (-1, -1):
        intersect := true;
      fi;
      j := j+1;
    endfor;
    if intersect = false: 
      random_vector[i] = candidate;
      drawarrow candidate withcolor random_color;
      freelabel(btex $v$ etex, point .5 of candidate, point .5 of candidate + dir (random_angle + 90));
      i := i+1;
    fi;
  endfor;
\stopMPcode


\stoptext

Answer 1

Removing the path variable bb[] (which is not used in the following code anyway, certainly a relic of my first attempts on this subject) makes it work with ConTeXt-MKIV. Maybe it was conflict detected with the bb() macro defined just after. But in this case, I've no idea about why it did work with direct MetaFun and not with ConTeXt.

Replace thus

 path candidate, random_vector[], bb_candidate, bb[];

by

path candidate, random_vector[] ;

(the bb_candidate path variable is of no use either).

Results:

EDIT I've just checked: while it works well now with ConTeXt-MKIV, it still doesn't work with ConTeXt-MKII. I'm at a loss for guessing why, since I'm not a regular ConTeXt user.

EDIT (bis) It seems that the actual reason it works now (at least with ConTeXt-MKIV) is that I've replaced the equation i = 1 by an assignment i := 1. See the comments below. I would suppose that the i variable is already defined somewhere else in ConTeXt-Metafun… But it still does not work with ConTeXt-MKII anyway. (No more error in that case, but the picture does not appear.)