docs: def parse_nested(
      bels: list, char_locs: CharLocs, parsed: Parsed, errors: Errors
  ) -> Tuple[Parsed, Errors]:
      """ Parse nested BEL object """
  
      for sp in char_locs[
          "nested_parens"
      ]:  # sp = start parenthesis, ep = end parenthesis
          ep, level = char_locs["nested_parens"][sp]
          if ep == -1:
              ep = len(bels) + 1
          parsed[(sp, ep)] = {"type": "Nested", "span": (sp, ep)}
  
      return parsed, errors