docs: def col_frequencies(col, weights=None, gap_chars='-.'):
      """Frequencies of each residue type (totaling 1.0) in a single column."""
      counts = col_counts(col, weights, gap_chars)
      # Reduce to frequencies
      scale = 1.0 / sum(counts.values())
      return dict((aa, cnt * scale) for aa, cnt in counts.iteritems())