[R] plot curve on surface

[Michael Meyer]

Greetings,

I would like to plot a curve (in color red) on a surface in a 3D plot.
Say

    x <- as.vector(seq(-1,1,0.2))
    dim(x) <- c(length(x),1)
    y <- as.vector(seq(-1,1,0.2))
    dim(y) <- c(1,length(y))
    
    z <- x%*%y         # (x_iy_j)

    # 3D plot of z(x,y)=xy:
    persp(x,y,z)
 
Now I would like to plot the curve z=z(x,x) in red on this surface.
How do I do that?
 
Many thanks,
 
Michael Meyer