% --- start of displayed preamble in the book ---

% --- start of displayed preamble in the book ---
def r(expr theta)=
(theta/10)
enddef;

vardef polcurve(text f)(expr c,a,b,n)=
save delta,_r,_a;
delta=(b-a)/n;
draw (c+f(a)*dir(a))
for i:=0 upto n-1:
hide(
_a:=a+i*delta;
_r:=f(_a);)
..(c+f(_a+delta)*dir(_a+delta))
endfor;
enddef;
% --- end of displayed preamble in the book ---
defaultfont:="ptmr8r";
warningcheck:=0;
beginfig(1)
polcurve(r)(origin,0,1150,1000);
endfig;
end;