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Source for wiki DatumLabels version 3
I tested datum-label support with the following expression:
(let ((x '(#1=(a b) #1#))) (eq? (car x) (cadr x)))
Gauche, MIT, Bigloo, Kawa, Chez, Ikarus/Vikare, Larceny^*^, Mosh, STklos, !FemtoLisp, Sagittarius, Foment, Picrin support the syntax and return `#t`;
Racket, Chicken, Scheme48/scsh, Guile, SISC, SCM, Larceny^*^, Ypsilon, !IronScheme, NexJ, JScheme, KSi, !SigScheme, !TinyScheme, Scheme 9, RScheme, S7, XLisp, Rep, Schemik, Elk, SXM, Sizzle, Dfsch, Inlab, Oaklisp, Owl Lisp, Chibi (native mode) report lexical syntax errors;
Gambit, Shoe, BDC, UMB, Llava, Chibi (R7RS mode) return `#f` for whatever reason.
^*^Larceny v0.98 normally recognizes the syntax, but reports a lexical error in R6RS mode (as is required by the R6RS).