I have an object for which equality tests take a long time, and I
have lots of others to compare it with.  It is
an instance variable of my class, and it is also accessble by
attr_accessor.  I'd like to hold a hash (number not Hash.new()) for
this object so I can compare it quickly with another.  How do I detect that
it has changed so that the hash gets updated correctly?  Does attr_accessor
construct code that leaves the right hooks in to detect this, or
must I write my own?  Should attr_accessor do that if it doesn't?
Can I trap this by interposing in freeze tests, somehow?

If this is a code smell, how should I handle frequently queried,
slow-to-test, but changing instance variables?  Make them all observables
and use update()? (That sounds expensive.)

        Thank you
        Hugh