Explain that "e if C" (without the "else" part) is a bad idea.

pep-0308.txt

@@ -110,6 +110,18 @@ Alternatives
     is called.  There's no way to short-circuit the expression
     evaluation.
 
+
+Variations
+
+    It has been proposed to make the 'else' part optional.  This would
+    be a really bad idea.  I showed:
+
+        x = e if C
+
+    to several people and they all thought that if C was false, it
+    would leave x unchanged.  So don't even think about this one!
+
+
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