Some containers become temporarily immutable during iteration. This includes
dicts, sets, and collections.deque. Their implementation is equally simple
-though they need to permantently set their length to zero whenever there is
+though they need to permanently set their length to zero whenever there is
an attempt to iterate after a length mutation.
The situation slightly more involved whenever an object allows length mutation
-during iteration. Lists and sequence iterators are dynanamically updatable.
+during iteration. Lists and sequence iterators are dynamically updatable.
So, if a list is extended during iteration, the iterator will continue through
the new items. If it shrinks to a point before the most recent iteration,
then no further items are available and the length is reported at zero.