Use a proper multiset for progression items

This can cut generation times in half in some cases

_vendor/collections_extended/__init__.py

@@ -0,0 +1,55 @@
+"""collections_extended contains a few extra basic data structures."""
+from ._compat import Collection
+from .bags import bag, frozenbag
+from .setlists import setlist, frozensetlist
+from .bijection import bijection
+from .range_map import RangeMap, MappedRange
+
+__version__ = '1.0.2'
+
+__all__ = (
+	'collection',
+	'setlist',
+	'frozensetlist',
+	'bag',
+	'frozenbag',
+	'bijection',
+	'RangeMap',
+	'MappedRange',
+	'Collection',
+	)
+
+
+def collection(iterable=None, mutable=True, ordered=False, unique=False):
+	"""Return a Collection with the specified properties.
+
+	Args:
+		iterable (Iterable): collection to instantiate new collection from.
+		mutable (bool): Whether or not the new collection is mutable.
+		ordered (bool): Whether or not the new collection is ordered.
+		unique (bool): Whether or not the new collection contains only unique values.
+	"""
+	if iterable is None:
+		iterable = tuple()
+	if unique:
+		if ordered:
+			if mutable:
+				return setlist(iterable)
+			else:
+				return frozensetlist(iterable)
+		else:
+			if mutable:
+				return set(iterable)
+			else:
+				return frozenset(iterable)
+	else:
+		if ordered:
+			if mutable:
+				return list(iterable)
+			else:
+				return tuple(iterable)
+		else:
+			if mutable:
+				return bag(iterable)
+			else:
+				return frozenbag(iterable)