efficient global variables in Common Lisp

 In Common Lisp, a special variable that is never dynamically bound typically
 serves as a stand-in for a global variable. This library provides true global
 variables that are implemented by some compilers. An attempt to rebind a
 global variable properly results in a compiler error. That is, a global
 variable cannot be dynamically bound.
 .
 Global variables therefore allow one to communicate an intended usage that
 differs from special variables. Global variables are also more efficient than
 special variables, especially in the presence of threads.