## Macro: COMMON-LISP:DEFPARAMETER
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> Define a parameter that is not normally changed by the program,
> but that may be changed without causing an error. Declare the
> variable special and sets its value to VAL, overwriting any
> previous value. The third argument is an optional documentation
> string for the parameter.

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