Hi,

after I have contributed two comparably trivial modules to Gnulib, I
would like to contribute a less trivial module this time. It
implements a persistent version of Phil Bagwell's HAMTs, which has
been popularized by Clojure. HAMTs can be used when a persistent
(functional/pure) version of a data structure akin to hash tables is
needed. For example, the dynamic environment of a (possibly
multi-threaded) Lisp or Scheme can be modeled with persistent HAMTs.

Please take a look at the attached patch.

Thank you,

Marc