 class=syntax-token>abstract</span> attribute, the function <span class=nonterminal>final-fn</span> is called when
the Caml block representing a value of this typedef becomes
unreachable from Caml and is reclaimed by the Caml garbage collector. 
Similarly, <span class=syntax-token>compare(</span><span class=nonterminal>compare-fn</span><span class=syntax-token>)</span> and <span class=syntax-token>hash(</span><span class=nonterminal>hash-fn</span><span class=syntax-token>)</span> attach a
comparison functio