How to use the ghstack.typing.GhNumber function in ghstack

title, pr_body = self._default_title_and_body(commit, None)

        # Determine the next available GhNumber.  We do this by
        # iterating through known branches and keeping track
        # of the max.  The next available GhNumber is the next number.
        # This is technically subject to a race, but we assume
        # end user is not running this script concurrently on
        # multiple machines (you bad bad)
        refs = self.sh.git(
            "for-each-ref",
            "refs/remotes/origin/gh/{}".format(self.username),
            "--format=%(refname)").split()
        max_ref_num = max(int(ref.split('/')[-2]) for ref in refs) \
            if refs else 0
        ghnum = GhNumber(str(max_ref_num + 1))

        # Create the incremental pull request diff
        tree = commit.patch.apply(self.sh, self.base_tree)

        # Actually, if there's no change in the tree, stop processing
        if tree == self.base_tree:
            self.ignored_diffs.append((commit, None))
            logging.warn("Skipping {} {}, as the commit has no changes"
                         .format(commit.oid, title))
            return

        assert ghnum not in self.seen_ghnums
        self.seen_ghnums.add(ghnum)

        new_pull = GitCommitHash(
            self.sh.git("commit-tree", tree,

If you think this is in error, edit the Summary in the Phabricator diff
to delete the line 'Pull Request resolved' and then run ghexport again.
''')
            else:
                raise RuntimeError('''\
This commit appears to already be associated with a pull request,
but the pull request doesn't look like it was submitted by ghstack.
If you think this is in error, run:

    ghstack unlink {}

to disassociate the commit with the pull request, and then try again.
(This will create a new pull request!)
'''.format(commit.oid))
        gh_number = GhNumber(m.group(1))

        # NB: Technically, we don't need to pull this information at
        # all, but it's more convenient to unconditionally edit
        # title/body when we update the pull request info
        title = r['title']
        pr_body = r['body']
        if self.update_fields:
            title, pr_body = self._default_title_and_body(commit, pr_body)

        # TODO: remote summary should be done earlier so we can use
        # it to test if updates are necessary
        remote_summary = ghstack.git.split_header(
            self.sh.git(
                "rev-list", "--max-count=1", "--header", "origin/" + branch_orig(self.username, gh_number)
            )
        )[0]

# Description of all the diffs we submitted; to be populated
    # by Submitter.
    stack_meta: List[DiffMeta]

    # List of input diffs which we ignored (i.e., treated as if they
    # did not exist on the stack at all), because they were associated
    # with a patch that contains no changes.  GhNumber may be false
    # if the diff was never associated with a PR.
    ignored_diffs: List[Tuple[ghstack.diff.Diff, Optional[GitHubNumber]]]

    # List of diffs to process, in chronological order
    stack: List[ghstack.diff.Diff]

    # Set of seen ghnums
    seen_ghnums: Set[GhNumber]

    # String used to describe the stack in question
    stack_header: str

    # Clobber existing PR description with local commit message
    update_fields: bool

    # Print only PR URL to stdout
    short: bool

    # Force an update to GitHub, even if we think that your local copy
    # is stale.
    force: bool

    # Do not skip unchanged diffs
    no_skip: bool