Add a note reminding the reader that sets are not sequences. I
received feedback that was based in the misunderstanding that sets
were sequences.
diff --git a/Lib/sets.py b/Lib/sets.py
index 2d44c28..4df30f8 100644
--- a/Lib/sets.py
+++ b/Lib/sets.py
@@ -4,6 +4,16 @@
ignored. The usual operations (union, intersection, deletion, etc.)
are provided as both methods and operators.
+Important: sets are not sequences! While they support 'x in s',
+'len(s)', and 'for x in s', none of those operations are unique for
+sequences; for example, mappings support all three as well. The
+characteristic operation for sequences is subscripting with small
+integers: s[i], for i in range(len(s)). Sets don't support
+subscripting at all. Also, sequences allow multiple occurrences and
+their elements have a definite order; sets on the other hand don't
+record multiple occurrences and don't remember the order of element
+insertion (which is why they don't support s[i]).
+
The following classes are provided:
BaseSet -- All the operations common to both mutable and immutable