def fa2s2b(fastas):
    """
    convert fastas to s2b dictionary
    """
    s2b = {}
    for fa in fastas:
        for seq in parse_fasta(fa):
            s = seq[0].split('>', 1)[1].split()[0]
            s2b[s] = fa.rsplit('/', 1)[-1].rsplit('.', 1)[0]
    return s2b