Prevent warnings about exception__prev from cexcept.h (maybe-uninitialized).
These might be selectively disabled, but the mechanism for doing so is not
obvious.
1a=4 2 3deff(x): 4ifnotx: 5defg(y): 6returnx,y,a 7returng# UnboundLocalError: not defined if x is true 8 9result=f(1)(2) 10assertresult==(1,2,4)