import graph3;  
import contour;  
import grid3;  
import palette;  

size(8cm,IgnoreAspect);  
 
currentprojection=orthographic(-10,-10,6);  
limits((0,0,0),(5,10,12));  
 
real f(pair z) {return (z.x+z.y)/(2+cos(z.x)*sin(z.y));}  
 
real[] levels={4,6};  
 
surface s=surface(f,(0,0),(5,10),120);  
 
s.colors(palette(s.map(new real(triple v) {return find(levels > v.z);}), 
Rainbow())); 
draw(s);  
 
grid3(new grid3routines [] {XYXgrid, ZXgrid(10), ZYgrid(5)},  
      Step=2,  
      step=1,  
      pGrid=new pen[] {red, blue, black},  
      pgrid=new pen[] {0.5red, lightgray, lightgray});  

xaxis3(Label("$x$",position=MidPoint,align=SE),  
       Bounds(Min,Min),  
       OutTicks());  
yaxis3(Label("$y$",position=MidPoint,align=SW),  
       Bounds(Min,Min),  
       OutTicks(Step=2));  
zaxis3(Bounds(Max,Both));
zaxis3(Label("$z$",position=EndPoint,align=N+W,p=red),  
       XYEquals(0,10),  
       InTicks(beginlabel=false,endlabel=false,Label(align=Y))  
       );  

draw(lift(f,contour(f,(0,0),(5,10),levels)),red);