docs: def tokenize(s):
      """ Tokenize a string by splitting it by + and -
  
      >>> tokenize('this + that')
      ['this', 'PLUS', 'that']
  
      >>> tokenize('this+that')
      ['this', 'PLUS', 'that']
  
      >>> tokenize('this+that-other')
      ['this', 'PLUS', 'that', 'MINUS', 'other']
      """
  
      # Crude tokenization
      s = s.replace('+', ' PLUS ').replace('-', ' MINUS ') \
          .replace('/', ' DIVIDE ').replace('*', ' MULTIPLY ')
      words = [w for w in s.split(' ') if w]
      return words