def twisted expr t of p =

def twisted expr t of p =
rotated angle direction arctime t of p of p
shifted point arctime t of p of p
enddef;
vardef rope expr c =
save s, w, hemp, n, a, b, A;
color hemp; hemp = 1/256 (192, 149, 82);
w = -1; n = -1; A = arclength c; s = A/floor(A/2);
path a[];
for t=0 step s until A + 1: a[incr n] =
(0,+w) twisted t-3/2s of c .. (0,+w) twisted t-1/2s of c ..
(0,-w) twisted t+1/2s of c .. (0,-w) twisted t+3/2s of c;
endfor
image(for i=1 upto n:
path b; b = buildcycle(a[i-1], reverse a[i]);
fill b withcolor 1/2[white, hemp];
draw b withpen pencircle scaled 1/8;
endfor)
enddef;