Use a proper multiset for progression items

This can cut generation times in half in some cases

_vendor/collections_extended/_util.py

@@ -0,0 +1,16 @@
+"""util functions for collections_extended."""
+
+
+def hash_iterable(it):
+	"""Perform a O(1) memory hash of an iterable of arbitrary length.
+
+	hash(tuple(it)) creates a temporary tuple containing all values from it
+	which could be a problem if it is large.
+
+	See discussion at:
+	https://groups.google.com/forum/#!msg/python-ideas/XcuC01a8SYs/e-doB9TbDwAJ
+	"""
+	hash_value = hash(type(it))
+	for value in it:
+		hash_value = hash((hash_value, value))
+	return hash_value