z0 <- c(-1,1); u <- rep(z0[1],4)
z <- array(u,dim=c(2,2)); x <- c(-1,1); y <- c(-1,1)
F <- persp(x,y,z,theta=20,phi=30,scale=TRUE,zlim=z0,xlim=x,ylim=y,d=1)
for (a in (0:50)*2*pi/50)
  lines(trans3d(c(cos(a),cos(a+pi)),c(sin(a),sin(a+pi)),c(-1,1),pmat=F),col="red")
a <- (0:50)*2*pi/50
lines(trans3d(cos(a),sin(a),1,pmat=F),col="blue")
lines(trans3d(cos(a),sin(a),-1,pmat=F),col="blue")

# per non avere il bordo della "scatola"
z0 <- c(-1,1); u <- rep(z0[1],4)
z <- array(u,dim=c(2,2)); x <- c(-1,1); y <- c(-1,1)
F <- persp(x,y,z,theta=20,phi=30,scale=TRUE,zlim=z0,xlim=x,ylim=y,d=1,box=FALSE,border="white")
for (a in (0:50)*2*pi/50)
  lines(trans3d(c(cos(a),cos(a+pi)),c(sin(a),sin(a+pi)),c(-1,1),pmat=F),col="red")
a <- (0:50)*2*pi/50
lines(trans3d(cos(a),sin(a),1,pmat=F),col="blue")
lines(trans3d(cos(a),sin(a),-1,pmat=F),col="blue")