# Description:
# Plots slices of the function f(x, y, z) = x^2 + y^2 - z.
# 
clear
# define function to plot
function f(x, y, z) begin 
	return x^2 + y^2 - z
end
# Number of points in x and y grids
N = 21 
# x ranges from -2 to 2
x = 4 * (seq(N) - 1) / real(N - 1) - 2 
# y ranges from -2 to 2
y = x' 
# x on 2 - dimensional grid
X = fillcols(x, N) 
# y on 2 - dimensional grid
Y = fillrows(y, N) 
#
# put title in viewport 0
gtitle("X^2 + Y^2 - Z") 
# define viewport 1
gaddview(0., .15, 1., .7) 
# contour levels
level = seq(5) - 1.5 
color = { "black", "red", "blue", "green", "black" }
# set colors
gcolor(color) 
gxtitle("X")
gytitle("Y")
gztitle("Z")
for z = 0 to 4 begin
# plot the contour
	contour(f(X, Y, z), level, x, y, z, "surface") 
end
grotate(30, 0, 70)