code: def calc_PnPs(counts):
      
      if counts['pos non-syn'] == 0 or counts['pos syn'] == 0:
          return 'n/a'
      NonSyn = float(counts['obs non-syn'])/float(counts['pos non-syn'])
      Syn    = float(counts['obs syn'])    /float(counts['pos syn'])
      if Syn == 0:
          return 'n/a'
      return NonSyn/Syn
docs: counts = {'obs syn':#, 'pos syn':#, 'obs non-syn':#, 'pos non-syn':#}