I'm learning closures, and it is stated that FnMut parameter must be declared mut. One example I find is the fold method of iterator. But when I look up the documentation, the closure parameter doesn't have mut (Iterator in std::iter - Rust) while the linked source code has mut (iterator.rs - source), and I notice the same thing for the self parameter. I'm a bit confused. Is it a bug in documentation or is it a convention to elide some finer details? As rust is new to me, I would prefer the documentation being more precise.

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