They'll be in the next draft, which will come out after SRFI 151 (the replacement for 142), which is almost done.

 

Frankly, my dear, I don't *care* about people who want to run Scheme on their Unisys ClearPath Dorado mainframes.  They typically pay per CPU cycle, anyway.  The IBM 360 (1964) and the DEC PDP-1 (1963) were already 2's complement when the remote ancestor of the Dorado, the Univac 1100, came out.

 

Backward compatibility with existing Schemes, especially R6RS.  N-ary arithmetic operations could provoke overflows at unspecified points as well, so they are not really associative.  I suppose the bitwise operators could be made n-ary.