docs: def symbols_to_prob(symbols):
      '''
      Return a dict mapping symbols to  probability.
  
      input:
      -----
          symbols:     iterable of hashable items
                       works well if symbols is a zip of iterables
      '''
      myCounter = Counter(symbols)
  
      N = float(len(list(symbols)))   # symbols might be a zip object in python 3
  
      for k in myCounter:
          myCounter[k] /= N
  
      return myCounter