I'm having a hard time understanding how exceptions work in the SXML
phase. Specifically, can I get an explanation of the fields of
Raise of {exn: VarExp.t, extend: bool}?
My current understanding is that exceptions are declared in decs of an
Exp, and any Raise must occur in the try field of a Handle. I'm not sure
of the purpose of extend. Is this on the right track?