Use a proper multiset for progression items

This can cut generation times in half in some cases

_vendor/collections_extended/_compat.py

@@ -0,0 +1,53 @@
+"""Python 2/3 compatibility helpers."""
+import sys
+
+is_py2 = sys.version_info[0] == 2
+
+if is_py2:
+	def keys_set(d):
+		"""Return a set of passed dictionary's keys."""
+		return set(d.keys())
+else:
+	keys_set = dict.keys
+
+
+if sys.version_info < (3, 6):
+	from collections import Sized, Iterable, Container
+
+	def _check_methods(C, *methods):
+		mro = C.__mro__
+		for method in methods:
+			for B in mro:
+				if method in B.__dict__:
+					if B.__dict__[method] is None:
+						return NotImplemented
+					break
+			else:
+				return NotImplemented
+		return True
+
+	class Collection(Sized, Iterable, Container):
+		"""Backport from Python3.6."""
+
+		__slots__ = tuple()
+
+		@classmethod
+		def __subclasshook__(cls, C):
+			if cls is Collection:
+				return _check_methods(C, "__len__", "__iter__", "__contains__")
+			return NotImplemented
+
+else:
+	from collections.abc import Collection
+
+
+def handle_rich_comp_not_implemented():
+	"""Correctly handle unimplemented rich comparisons.
+
+	In Python 3, return NotImplemented.
+	In Python 2, raise a TypeError.
+	"""
+	if is_py2:
+		raise TypeError()
+	else:
+		return NotImplemented