updated contents from Atlas repo

iterables/simplest_generators.doctest

@@ -0,0 +1,42 @@
+>>> def gen_123():
+...     yield 1
+...     yield 2
+...     yield 3
+...
+>>> gen_123  # doctest: +ELLIPSIS
+<function gen_123 at 0x...>
+>>> gen_123()   # doctest: +ELLIPSIS
+<generator object gen_123 at 0x...>
+>>> for i in gen_123():
+...     print(i)
+1
+2
+3
+>>> g = gen_123()
+>>> next(g)
+1
+>>> next(g)
+2
+>>> next(g)
+3
+>>> next(g)
+Traceback (most recent call last):
+  ...
+StopIteration
+
+
+>>> def gen_AB():
+...     print('start')
+...     yield 'A'
+...     print('continue')
+...     yield 'B'
+...     print('end.')
+...
+>>> for c in gen_AB():
+...     print('--->', c)
+...
+start
+---> A
+continue
+---> B
+end.