def _s2t_idx(s_idx, sV, tV, oracle=None):
    """
    Translates an feature-index from the source domain to the feature-index of the target domain.
    If the source and target share the language, the vocabularies are the only data structures involved.
    If otherwise, the oracle is used to translate the word before retrieving the index.
    """
    t_idx = None
    word = sV.idx2word(s_idx)
    if oracle:
        word = oracle.source2target(word)
    if word:
        t_idx = tV.word2idx(word)
    return t_idx