r.star<-function(r){
#function to produce the reverse comp. of r.
#r is a string (vector) of length n
b<-c(4:1) #complements of 1,2,3,4
r.rev<-r[length(r[]):1] #reverses sequence
s<-r.rev
s[]<-b[r.rev[]] #replaces w/ complementary "letters"
return(s)
}