explain why adding a new builtin won't work.

pep-0308.txt

@@ -98,6 +98,15 @@ Alternatives
     advantage of evaluating strictly from left to right (not that that
     is a requirement for being Pythonic -- list comprehensions don't).
 
+    Many people suggest adding a new builtin instead of extending the
+    syntax of the language, e.g.:
+
+        ifelse(condition, expression1, expression2)
+
+    This won't work the way a syntax extension will because both
+    expression1 and expression2 must be evaluated before the function
+    is called.  There's no way to short-circuit the expression
+    evaluation.
 
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