Dear Caml riders,

I found by chance the "The invert Benchmark"
(http://www.lib.uchicago.edu/keith/crisis/benchmarks/invert/).  As you
will notice the Caml code (even compiled) performs poorly.  I guess
part of the problem is due to using Map when Hashtbl is more suited.
So I tried to rewrite the code using Hashtbl (attached to this mail).
What I got some trouble to figure out is how to get a list of the keys
where each of the keys appears only once.  I eventually went the easy
way.  Anybody got better ideas to improve efficiency?  Could a "keys"
function be an interesting addition to Hashtbl???

Another related question that popped up is: how to _efficiently_
implement a join operation (join : string -> string list -> string is
defined by: join c [s1;...;sn] = s1 ^ c ^ ... ^ c ^ sn) ?

Cheers,
ChriS