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Source for wiki SetUndefinedVariable version 3
== Attempting to set an unbound variable==
Racket, Gauche, MIT, Gambit, Bigloo, Scheme48/scsh, Guile, SISC, Chibi, SCM, Ypsilon, NexJ, STklos, KSi, !SigScheme, Shoe, !TinyScheme, Scheme 9, Dream, S7, Schemik, Elk, UMB, Sizzle, Spark, Dfsch, Inlab, Sagittarius complain about the attempt to use `set!` on an unbound variable.
Chicken, Kawa, Chibi, Chez, !Ikarus/Vicare, Larceny, Mosh, XLisp, Rep, VX, Llava, Femtolisp do not.
Oaklisp, SXM print a warning but allow it.
Owl Lisp doesn't have variable assignment.