Function code_at 

pub fn code_at(
    secret_b32: &str,
    params: &Params,
    unix_secs: u64,
) -> Result<String, String>

The code a stored seed produces at a given time.

For an Hotp seed the time is ignored and params.counter is used, so this is still the single entry point for every kind and no caller has to know which it holds.