Added brief section about exception propagation.

pep-0255.txt

@@ -169,6 +169,36 @@ Specification
     functions.
 
 
+Generators and Exception Propagation
+
+    If an unhandled exception-- including, but not limited to,
+    StopIteration --is raised by, or passes through, a generator function,
+    then the exception is passed on to the caller in the usual way, and
+    subsequent attempts to resume the generator function raise
+    StopIteration.  In other words, an unhandled exception terminates a
+    generator's useful life.
+
+    Example (not idiomatic but to illustrate the point):
+
+    >>> def f():
+    ...     return 1/0
+    >>> def g():
+    ...     yield f()  # the zero division exception propagates
+    ...     yield 42   # and we'll never get here
+    >>> k = g()
+    >>> k.next()
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+      File "<stdin>", line 2, in g
+      File "<stdin>", line 2, in f
+    ZeroDivisionError: integer division or modulo by zero
+    >>> k.next()  # and the generator function cannot be resumed
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    StopIteration
+    >>>
+
+
 Example
 
         # A binary tree class.