docs: def identity_avg(aseq, bseq):
      """Compute absolute identity (# matching sites) between sequence strings."""
      match = identity_abs(aseq, bseq)
      alen = len(aseq.replace('-', '').replace('.', ''))
      blen = len(bseq.replace('-', '').replace('.', ''))
      avg_len = 0.5 * (alen + blen)
      return match / avg_len