% --- start of displayed preamble in the book ---
input featpost3Dplus2D
% --- end of displayed preamble in the book ---
% graphic converted to gray in book using 'color2gray'
defaultfont:="ptmr8r";
warningcheck:=0;
beginfig(1)
def zsurface( expr xc, yc ) =
cosd(xc*57)*cosd(yc*57)
+4*mexp(-(xc**2+yc**2)*6.4)
enddef;