I have a string "ECET" and I would like to create all the possible strings where I substitute one or more letters (all but the first) with "X". 

So in this case my result would be:

```
> result
[1] "EXET" "ECXT" "ECEX" "EXXT" "EXEX" "ECXX" "EXXX"

```

Any ideas as to how to approach the issue?

This is not just create the possible combinations/permutations of "X" but also how to combine them with the existing string.