In Python, I can do:

```
from itertools import product

k = 3
for kmer in product("AGTC", repeat=k):
    print(kmer)

```

In Rust, I can force the behavior of `k=3` by:

```
#[macro_use] extern crate itertools;

for kmer in iproduct!("AGTC".chars(), "AGTC".chars(), "AGTC".chars()){
    println!("{:?}", kmer);
}

```

But what if I wanted `k=4` or `k=5`?