give equivalent code for raise from

pep-0415.txt

@@ -50,6 +50,12 @@ printed. ``__cause__`` will return to its original purpose and values.
 There is precedence for ``__suppress_context__`` with the
 ``print_line_and_file`` exception attribute.
 
+To summarize, ``raise exc from cause`` will be equivalent to::
+
+    exc.__cause__ = cause
+    exc.__suppress_context__ = cause is None
+    raise exc
+
 Patches
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