Evaluation
The evaluation of a rest $error Q returns an error
$error(Ge), where Ge is the result of evaluating the path
Q.
Env,0,St |- Q => Ge,St'
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Env,m,St |- $error Q => $error(Ge),St'
Env,0,St |- Q => $fail(0),St'
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Env,m,St |- $error Q => $error(Fname "Unexpected fail"),St'
Fname is the name of the function in which
the construct appears.
Env,0,St |- Q => $error(Ge),St'
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Env,m,St |- $error Q => $error(Ge),St'