Why would you extend rather than subclass the Array class? It kind of confirms all of my worst fears about Ruby's too-easy class reopening. (what happens when someone else defines an Array method called "categorize" for a totally unrelated purposes?)
I think that the Python syntax for this is
h[x] = h.get(x, 0) + 1
It isn't quite as concise as the Common Lisp but more so than Arc. I'd be curious to see what the Common Lisp looks like if you are doing something more complicated than an in-place increment. E.g. the equivalent of:
A hashtable containing integer values is a common implementation for the collection data structure known as a Bag or Counted Set. The value indicates how many instances of the key appear in the collection. Incrementing the value would be equivalent to adding a member instance. Giving a zero default is a shortcut to avoid having to check for membership.