docs: def _next_lexem(self, lexem_type, source_code, source_code_size):
          """Return next readable lexem of given type in source_code.
          If no value can be found, the neutral_value will be used"""
          # define reader as a lexem extractor
          def reader(seq, block_size):
              identificator = ''
              for char in source_code:
                  if len(identificator) == self.idnt_values_size[lexem_type]:
                      yield self.table_values[lexem_type][identificator]
                      identificator = ''
                  identificator += char
          lexem_reader = reader(source_code, self.idnt_values_size)
          lexem = None
          time_out = 0
          while lexem == None and time_out < 2*source_code_size: 
              lexem = next(lexem_reader)
              time_out += 1
          # here we have found a lexem
          return lexem