Given b => 312, what is the value of (a .,b)?
Guile, Bigloo, Shoe, Dream, S7, Elk, Sizzle, Dfsch: (a .,b), treating .,b as a symbol.
Racket, Gauche, MIT, Scheme48/scsh, Kawa, Chibi, SCM, Chez, STklos, KSi, Scheme 9, BDC, Rep, Schemik, Oaklisp, FemtoLisp, Inlab, Owl Lisp, Sagittarius (and Common Lisp): (a . 312).
Chicken: (a |.| 312).
SigScheme (which explicitly calls it a compatibility issue), Vicare, Ypsilon, Mosh, and probably the other R6RS systems: syntax error.
TinyScheme, XLisp, SXM: no support for quasiquote.