code: def _next_condition_lexems(self, source_code, source_code_size):
          
          # find three lexems
          lexems = tuple((
              self._next_lexem(LEXEM_TYPE_COMPARISON, source_code, source_code_size),
              self._next_lexem(LEXEM_TYPE_OPERATOR  , source_code, source_code_size),
              self._next_lexem(LEXEM_TYPE_COMPARISON, source_code, source_code_size)
          ))
          # verify integrity
          if None in lexems: # one of the condition lexem was not found in source code 
              return None
          else: # all lexems are valid
              return ' '.join(lexems)
docs: Return condition lexem readed in source_code