ploting "contour plot" and "2D plot"



Hello, 


Thank you for helping me the last time with plotting the roots of polynomials. I come with another question... I would like to plot a "contour plot" and a "2D plot" on the same figure.

For example:
---------------

contour_plot(y^2+x^2, [x,-5,5], [y,-5,5]);
plot2d(x+2,[x,-5,5], [y,-5,5]);
----------------

This is to illustrate an optimization problem using Lagrange multipliers


Like this plot:
http://commons.wikimedia.org/wiki/File:LagrangeMultipliers2D.svg


Is it possible to do the same illustration in 3D? Like this one:

http://commons.wikimedia.org/wiki/File:LagrangeMultipliers3D.png


Thanks for your help!

Samy