If any rules contain paths, then we reject any rules that don't contain paths. Allowing a mix leads to strange semantics, since the path-based rules only match things where the path refers to semantically the same thing, whereas the non-path-based rules could match anything. Specifically, if we have a rule like `foo ==>> bar` we only want to match the `foo` that is in the current scope, not any `foo`. However "foo" can be parsed as a pattern (BIND_PAT -> NAME -> IDENT). Allowing such a rule through would result in renaming everything called `foo` to `bar`. It'd also be slow, since without a path, we'd have to use the slow-scan search mechanism.
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