code: def loc(parser, start):
      # type: (Parser, int) -> Optional[Loc]
      
      if parser.options["no_location"]:
          return None
  
      if parser.options["no_source"]:
          return Loc(start, parser.prev_end)
  
      return Loc(start, parser.prev_end, parser.source)
docs: Returns a location object, used to identify the place in
      the source that created a given parsed object.