code: def _check_flow_operator(string):
      
      greater_than_count = string.count('>')
      if greater_than_count > 1:
          raise ValueError(MULTIPLE_FLOW_OPERATORS)
      elif (string[0] == '>') or (string[-1] == '>'):
          raise ValueError(FLOW_OPERATOR_ON_ENDS)
      else:
          if greater_than_count == 1:
              ret = True
          else:
              ret = False
      return ret
docs: Checks the flow operator ('>') to make sure that it:
      1) Is non empty
      2) There is only one of them