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.
