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It is not always desirable or possible to allow primitive procedures
to be redefined.

Compilation with exections is no more difficult when compiling with
signalled errors. A call to `error' (or whatever the error message
procedure might be) is simply replaced with a call to `raise'.

Whether the meaning of a compiled procedure changes obviously depends
on how you assign meaning. Given the same input, a compiled procedure
would either return the same value or raise the same
exception. Without exceptions, a compiled procedure for the same input
either returns the same value or signals an anonymous error. With
exceptions, equivalence between (compiled) procedures must take into
account the kind of exception that is raised, and this is exactly what
exceptions are for. The way in which the exception is handled is not
part of the meaning of a compiled procedure.