Highlight Features with Meshes
Mesh lines can be used to show constant curves on the plot of a function.
Show the curves where 
and 
are constant in black and white, respectively.
Show the curves where 
and 
are constant in black and white, respectively.
It is said that 
is a saddle point of 
if 
and 
. Saddle points occur in the following plot at 
, where the curves of constant 
cross.
