## Macro: COMMON-LISP:DEFVAR
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> Define a special variable at top level. Declare the variable
> SPECIAL and, optionally, initialize it. If the variable already has a
> value, the old value is not clobbered. The third argument is an optional
> documentation string for the variable.

### Lambda list
```cl
(sb-debug:var &optional sb-impl::val sb-impl::doc)
```