sim <- function(n,sigma=2.5) {
   require(FOCI)
   x1 <- rnorm(n)
   x2 <- x1 + 2.0*rnorm(n)
   x3 <- rnorm(n)
   x4 <- x3 + 1.5 *rnorm(n)
   x5 <- x1 + rnorm(n)/1.3
   x6 <- x2 + 2.25*rnorm(n)
   x7 <- x3 + x4 + 2.5 *rnorm(n)
   x8 <- x7 + 4.0 * rnorm(n)
   y <- x1 + x2 + 0.5*x3 + 0.4*x7 + sigma * rnorm(n)
   list(x=cbind(x1,x2,x3,x4,x5,x6,x7,x8),y=y)
}