[racket] empty hash tables

At Mon, 30 May 2011 10:24:20 +0200, Pierpaolo Bernardi wrote:
> BTW, since the topic of empty tables has been brought up: I am often
> (mildly) annoyed by the fact that make-hash takes 0 or 1 argument,
> while make-immutable-hash takes only 1 argument, forcing me to write
> (make-immutable-hash '())..
> 
> Is there a reason for this lack of simmetry?

I'll change `make-immitable-hash{,eq,eqv}' to make the argument
optional.