docs: def log_transform(rates):
      """ log transform a numeric value, unless it is zero, or negative
      """
      
      transformed = []
      for key in ['missense', 'nonsense', 'splice_lof', 'splice_region',
              'synonymous']:
          
          try:
              value = math.log10(rates[key])
          except ValueError:
              value = "NA"
          except KeyError:
              continue
          
          transformed.append(value)
      
      return '\t'.join(map(str, transformed))