CLHS: Section 6.1.9

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## 6.1.9 Notes about Loop

Types can be supplied for loop variables. It is not necessary to supply a type for any variable, but supplying the type can ensure that the variable has a correctly typed initial value, and it can also enable compiler optimizations (depending on the implementation).

The clause `repeat` n ... is roughly equivalent to a clause such as

     (loop for internal-variable downfrom (- n 1) to 0 ...)

but in some implementations, the `repeat` construct might be more efficient.

Within the executable parts of the loop clauses and around the entire loop form, variables can be bound by using let.

Use caution when using a variable named `IT` (in any package) in connection with loop, since `it` is a loop keyword that can be used in place of a form in certain contexts.

There is no standardized mechanism for users to add extensions to loop.

The following X3J13 cleanup issue, not part of the specification, applies to this section:

- LOOP-MISCELLANEOUS-REPAIRS:FIX

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