#Generates n observations from a normal distribution by using the acceptance
#method with a half-normal and exponential distribution.  Outputs the sample
# and two graphs.  A normal QQ plot is produced by default.
halfnorm=function(n,mean=0,sigma=1,pl=T){
x1=NULL
while(length(x1)<=n){
u=runif(1)
v=runif(1)
x=-log(u)
ex=exp(-(x^2+1)/2)
x1=c(x1,x[(v*u<=ex)]*(2*(u*v>=ex/2)-1))
}
x2=mean+sigma*x1
if (pl==T){
win.graph()
qqnorm(x2)
qqline(x2)
}
x2
}