Define THROW_EXCEPTION(E) which takes an exception class instance,
and uses it to construct an instance of a class which is a subclass
of E and ExceptionMixin. The original instance is discarded, and
the newly constructed sub-class is thrown. Equivalent to
"throw E;".
Define THROW_EXCEPTION2(ETYPE,MSG) which takes an exception class
type, and argument. Directly constructs a ExceptionMixin sub-class
with the given message argument. Equivalent to
"throw ETYPE(MSG);".
Define PRINT_EXCEPTION2(E, FP) If E is a instance of a sub-class of
ExceptionMixin then write information to FP (FILE*).
Define SHOW_EXCEPTION(E) If E is a instance of a sub-class of
ExceptionMixin then return a std::string with information.
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