docs: def letter_scales(counts):
      """Convert letter counts to frequencies, sorted increasing.""" 
      try:
          scale = 1.0 / sum(counts.values())
      except ZeroDivisionError:
          # This logo is all gaps, nothing can be done
          return []
      freqs = [(aa, cnt*scale) for aa, cnt in counts.iteritems() if cnt]
      freqs.sort(key=lambda pair: pair[1])
      return freqs