I think the intent of this code is perfectly obvious</sarcasm>

greycode.R

greycode <- function(p) 
{
	A <- matrix(c(0,1),2,1)
	if (p!=1) {
		for (i in 2:p) {
			P <- nrow(A)
			inds <- rev(1:P)
			A1 <- matrix(c(rep(0,P),rep(1,P)),2*P,1)
			A2 <- rbind(A,matrix(A[inds,],nrow(A),ncol(A)))
			A <- cbind(A1,A2)
		}
	}
	return(A)
}