docs: def reference(self, t, i, in_group=False):
          """Handle references."""
  
          current = []
  
          if not in_group and t == "m":
              current.append(self._re_start_wb)
          elif not in_group and t == "M":
              current.append(self._re_end_wb)
          elif not in_group and t == "R":
              current.append(self._re_line_break)
          elif not in_group and t == "X":
              no_mark = self.unicode_props("^m", None, in_group=False)[0]
              mark = self.unicode_props("m", None, in_group=False)[0]
              current.extend(self._grapheme_cluster % (no_mark, mark, mark))
          elif t == "e":
              current.append(self._re_escape)
          elif t == "l":
              current.extend(self.letter_case_props(_LOWER, in_group))
              self.found_property = True
          elif t == "L":
              current.extend(self.letter_case_props(_LOWER, in_group, negate=True))
              self.found_property = True
          elif t == "c":
              current.extend(self.letter_case_props(_UPPER, in_group))
              self.found_property = True
          elif t == "C":
              current.extend(self.letter_case_props(_UPPER, in_group, negate=True))
              self.found_property = True
          elif t == 'p':
              prop = self.get_unicode_property(i)
              current.extend(self.unicode_props(prop[0], prop[1], in_group=in_group))
              if in_group:
                  self.found_property = True
          elif t == 'P':
              prop = self.get_unicode_property(i)
              current.extend(self.unicode_props(prop[0], prop[1], in_group=in_group, negate=True))
              if in_group:
                  self.found_property = True
          elif t == "N":
              text = self.get_named_unicode(i)
              current.extend(self.unicode_name(text, in_group))
              if in_group:
                  self.found_named_unicode = True
          else:
              current.extend(["\\", t])
          return current