How to use the emmett.orm.migrations.exceptions.RevisionError function in emmett

if check:
                per_target = set()

            while todo:
                rev = todo.pop()
                if check:
                    per_target.add(rev)

                if rev in seen:
                    continue
                seen.add(rev)
                todo.extend(
                    map_[rev_id] for rev_id in fn(rev))
                yield rev
            if check and per_target.intersection(targets).difference([target]):
                raise RevisionError(
                    "Requested revision %s overlaps with "
                    "other requested revisions" % target.revision)

"argument '%(head_arg)s'; please "
                    "specify a specific target revision, "
                    "'@%(head_arg)s' to "
                    "narrow to a specific head, or 'heads' for all heads")
            multiple_heads = multiple_heads % {
                "head_arg": end or mh.argument,
                "heads": str(mh.heads)
            }
            raise Exception(multiple_heads)
        except ResolutionError as re:
            if resolution is None:
                resolution = "Can't locate revision identified by '%s'" % (
                    re.argument
                )
            raise Exception(resolution)
        except RevisionError as err:
            raise Exception(err.args[0])

"Revision %s is not an ancestor of revision %s" %
            (lower or "base", upper or "base")
        )


class MultipleHeads(RevisionError):
    def __init__(self, heads, argument):
        self.heads = heads
        self.argument = argument
        super(MultipleHeads, self).__init__(
            "Multiple heads are present for given argument '%s'; "
            "%s" % (argument, ", ".join(heads))
        )


class ResolutionError(RevisionError):
    def __init__(self, message, argument):
        super(ResolutionError, self).__init__(message)
        self.argument = argument

"""
    emmett.orm.migrations.exceptions
    --------------------------------

    Provides exceptions for migration operations.

    :copyright: (c) 2014-2019 by Giovanni Barillari
    :license: BSD, see LICENSE for more details.
"""


class RevisionError(Exception):
    pass


class RangeNotAncestorError(RevisionError):
    def __init__(self, lower, upper):
        self.lower = lower
        self.upper = upper
        super(RangeNotAncestorError, self).__init__(
            "Revision %s is not an ancestor of revision %s" %
            (lower or "base", upper or "base")
        )


class MultipleHeads(RevisionError):
    def __init__(self, heads, argument):
        self.heads = heads
        self.argument = argument
        super(MultipleHeads, self).__init__(
            "Multiple heads are present for given argument '%s'; "
            "%s" % (argument, ", ".join(heads))

class RevisionError(Exception):
    pass


class RangeNotAncestorError(RevisionError):
    def __init__(self, lower, upper):
        self.lower = lower
        self.upper = upper
        super(RangeNotAncestorError, self).__init__(
            "Revision %s is not an ancestor of revision %s" %
            (lower or "base", upper or "base")
        )


class MultipleHeads(RevisionError):
    def __init__(self, heads, argument):
        self.heads = heads
        self.argument = argument
        super(MultipleHeads, self).__init__(
            "Multiple heads are present for given argument '%s'; "
            "%s" % (argument, ", ".join(heads))
        )


class ResolutionError(RevisionError):
    def __init__(self, message, argument):
        super(ResolutionError, self).__init__(message)
        self.argument = argument

len(total_space.intersection(rev._all_nextrev)) > 1
        )

        # it's not possible for any "uppers" to be in branch_todo,
        # because the ._all_nextrev of those nodes is not in total_space
        #assert not branch_todo.intersection(uppers)

        todo = deque(
            r for r in uppers if r.revision in total_space)

        # iterate for total_space being emptied out
        total_space_modified = True
        while total_space:

            if not total_space_modified:
                raise RevisionError(
                    "Dependency resolution failed; iteration can't proceed")
            total_space_modified = False
            # when everything non-branch pending is consumed,
            # add to the todo any branch nodes that have no
            # descendants left in the queue
            if not todo:
                todo.extendleft(
                    sorted(
                        (
                            rev for rev in branch_todo
                            if not rev._all_nextrev.intersection(total_space)
                        ),
                        # favor "revisioned" branch points before
                        # dependent ones
                        key=lambda rev: 0 if rev.is_branch_point else 1
                    )