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It is not very common, but sometimes, $Failed is used as a head, like f[x___]:= $Failed[x] This makes it possible to have "return code" returned, rather than just a fact of failure. Basically, when this is used, it is usually in the error-reporting fall-back rule. In some cases, one may want to not evaluate the arguments x (e.g. if f is Hold*). I don't ...
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