If you think of the . here as Haskell's cons (:) then it makes sense. All the same, (x y . zs) can mean the exact same thing, not to mention it is shorter and familiar.
Arc uses the PLT Scheme reader. The PLT Scheme reader has a non-standard (as compared to RnRS) extension, namely the double-dot notation. The double-dot notation is used to write "infix expressions".
Consider:
(x . < . y) is turned into (< x y)
(integer? boolean? . -> . void?) is turned into (-> integer? boolean? void?)
This reader extension can be turned off, but setting the appropriate parameter.
That is strange, and probably a bug. You can use up to 2 dots and the last dot is handled first, consing x and (z . nil) then consing y and (x (z . nil)). '(w x . y . z) gives (y w x z).
I can almost see how this bug appeared. Searching for the first part of a dotted expression yields (w x), and then searching for the 2nd part yields z. For some reason the expression becomes '(y . (w x z)); that I cannot explain without looking at the code.
