+from itertools import filterfalse
+
+
+# https://docs.python.org/3/library/itertools.html#itertools-recipes
+
+def unique_everseen(iterable, key=None):
+ "List unique elements, preserving order. Remember all elements ever seen."
+ # unique_everseen('AAAABBBCCDAABBB') --> A B C D
+ # unique_everseen('ABBCcAD', str.lower) --> A B C D
+ seen = set()
+ seen_add = seen.add
+ if key is None:
+ for element in filterfalse(seen.__contains__, iterable):
+ seen_add(element)
+ yield element
+ else:
+ for element in iterable:
+ k = key(element)
+ if k not in seen:
+ seen_add(k)
+ yield element
+