contour plot



Hi, 

I woluld like to make contour plots of real part of 6-th iteration of 
complex number, like fig 5 in paper :
 "Real and imaginary parts of polynomial iterates " by  Julia A. Barnes, 
Clinton P. Curry
nyjm.albany.edu/j/2010/16-29v.pdf

or 
http://en.wikibooks.org/wiki/File:Julia_dendrit_modified.png

I have made some code :
-------------------
f(x,y):=block(
[z,c,nMax],
nMax:6,
c:%i,
z:x+y*%i,
for n:1 thru nMax step 1 do z:z*z+c,
return(imagpart(z))
);

contour_plot (f(x,y), [x, -4, 4], [y, -4, 4],[adapt_depth, 10],
[nticks,50])$
--------------------

But the result is not good.
Can I do it better ?

Thx in advance

Adam