class DataTree:
    ...

    def __getitem__(self, key: str) -> DataTree | DataArray:
        """
        Accepts node/variable names, or file-like paths to nodes/variables (inc. '../var').

        (Also needs to accommodate indexing somehow.)
        """
        ...

    def subset(self, keys: Sequence[str]) -> DataTree:
        """
        Return new tree containing only nodes with names matching keys.

        (Could probably be combined with `__getitem__`. 
        Also unsure what the return type should be.)
        """
        ...

    @property
    def subtree(self) -> Iterator[DataTree]:
        """An iterator over all nodes in this tree, including both self and all descendants."""
        ...

    def filter(self, filterfunc: Callable) -> Iterator[DataTree]:
        """Filters subtree by returning only nodes for which `filterfunc(node)` is True."""
        ...