It is simple to set a class constant in ruby:

  class Foo
    CLASS_CONSTANT = "never to change"
  end

And accessed like this:
  Foo::CLASS_CONSTANT

However, I'd like to be able to create a class variable that is easily
accessible (read/write) by outside users without having to write class
accessor methods.  How can I do this?

A really ugly way to set the variable without an accessor would be
this:
  class Foo; @@class_constant = "newvalue" end
but that is write only.

The syntax:
  Foo.class_constant = "newvalue"
would be preferable, but how to do this without writing accessor
methods?

Thanks