[Python-3000] Special object to return from functions that return only None

[Edward C. Jones]

Wild idea.

When I was a newbie, I repeatedly make the mistake of writing

alist = alist.sort()

I suggest a singleton object "UseForbidden" ("Py_UseForbidden" in C). 
"UseForbidden" can be used only to return from a function. Any other 
attempt to use "UseForbidden" raises an exception. The incorrect code 
above would give the error message "cannot assign to result of function".