## Macro: COMMON-LISP:DEFCONSTANT
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> Define a global constant, saying that the value is constant and may be
> compiled into code. If the variable already has a value, and this is not
> EQL to the new value, the code is not portable (undefined behavior). The
> third argument is an optional documentation string for the variable.

### Lambda list
```cl
(sb-impl::name sb-impl::value &optional sb-impl::doc)
```