[[1]]
[1] "a" "d"

[[2]]
[1] "a" "c"

[[3]]
[1] "d" "e"

[[4]]
[1] "e" "f"

[[5]]
[1] "b" "c"

[[6]]
[1] "f" "c"

[[7]]
[1] "c" "e"

[[8]]
[1] "f" "b"

[[9]]
[1] "b" "a"

for(i in 1:length(T){
   if (!( *first letter* %in% T && *second letter* %in% T)) {
      T[[i]] <- c(*first letter*, *second letter*)
   }
}