Function align 

pub fn align<A>(
    chain: &AlignChain,
    dense: &DenseFst<'_, A>,
) -> Result<Option<Alignment>, OpenFstError>
where
    A: Arc,
    A::Weight: FromScore,

Aligns chain to dense: the best path of the chain against the acoustic scores, exactly.

This is best_path over AlignChain::against, read back into phones. A caller who wants the path itself, either to interpret the transitions their own way or because they have replaced the chain with a topology of their own, should call those two directly; see trellis.

Returns None when no path exists, which means a reference longer than the audio with no skips to make up the difference.

Errors

A phone naming a column the acoustic model does not have, which is a mismatch between the reference and the model rather than a bad alignment; or a matrix so large that the traceback plane does not fit in memory, reported rather than attempted.