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The letrec namebinding construct
The letrec name binding construct allows for definition of mutually recursive functions
(letrec ((n<sub>1</sub> e<sub>1</sub>) ... (n<sub>k</sub> e<sub>k</sub>)) body-expr)
(letrec ((f1 (lambda (...) ... (f2 ...)))
         (f2 (lambda (...) ... (f1 ...)))