docs: def import_extension(dest, src, name, api=None, profile=None,
                       filter_symbol=None):
      """Imports Extension `name`, and all its dependencies.
  
      :param Registry dest: Destination Registry
      :param Registry src: Source Registry
      :param str name: Name of Extension to import
      :param str api: Prefer to import types with API name `api`,
                  or None to prefer Types with no API name.
      :type api: str
      :param filter_symbol: Optional symbol filter callable
      :type filter_symbol: Callable with signature
                           ``(symbol_type:str, symbol_name:str) -> bool``
      """
      if filter_symbol is None:
          filter_symbol = _default_filter_symbol
      ext = src.extensions[name] if isinstance(name, str) else name
      for req in ext.get_requires(api, profile):
          for x in req.types:
              if not filter_symbol('type', x):
                  continue
              import_type(dest, src, x, api, filter_symbol)
          for x in req.enums:
              if not filter_symbol('enum', x):
                  continue
              import_enum(dest, src, x)
          for x in req.commands:
              if not filter_symbol('command', x):
                  continue
              import_command(dest, src, x, api, filter_symbol)
      dest.extensions[name] = ext