[NTG-context] Metapost: draw only subpath; changing thickness of line

[Emanuel Han via ntg-context <ntg-context@ntg.nl>]

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Hi all,

I have this MWE. Now I want to draw only the subpath of S between gong6 and gong7 with a red line which is narrower at the two ends and thicker in the middle part. How to achieve this?
Thanks!
Emanuel

\setuppapersize[A4,landscape]
\starttext
\startMPcode
numeric u; u=1cm;
%equiangular spiral
numeric a, k ;
a = 2.3; %scaling factor
k = 8.61722335;
path S;
S = right for t=1 upto 360: .. a ** (t/50.2) * dir (-k*t) endfor;
draw S;

z1 = 8 dir -14;
path A; A = origin -- z1 scaled 0.7u ; draw A;
pair AEndPoint; AEndPoint = point 1 of A;
pair gong[], times; numeric n; n = 0;
for i = 1 upto length(A):
for j = 1 upto length(S):
times := subpath (i-1,i) of A intersectiontimes subpath (j-1,j) of S;
if xpart times > -1:
gong[incr n] = 1/2[point xpart times of subpath (i-1,i) of A,
point ypart times of subpath (j-1,j) of S];
fi
endfor
endfor

dotlabel.rt ("$gong6$", gong6);
dotlabel.rt ("$gong7$", gong7);

%Now I want to draw only the subpath of S between gong6 and gong7 with a red line which is narrower at the two ends and thicker in the middle part
\stopMPcode
\stoptext

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