[plt-scheme] inheritance of private fields?

Hello all:

I want to define a general class with some private fields that can be
used by subclass.  Here's a dumb example of what I thought would be the
right way to do this, but apparently isn't:

(define size% 
  (class* object% ()
    (public print-name print-age print-weight)

    (init-field (name 'Fred) (bday 070661)))

    (define age (calculate-age bday))
    (define weight 0)

    (define print-name (display name))
    (define print-age (display age))
    (define print-weight (display weight))

    (super-new)))

Now I want to be able to define a sub-class of this, so that age and
weight are available to definitions in that sub-class.  I haven't been
able to do that, so feel I must be missing something.

In the example age is a calculated value that depends on other
init-field values.  I'd make it public by putting it into the init-field
sequence, if there's a guarantee that the init-field entries are
processed in order (that it's a let* and not a let).  I can't find any
such guarantee in the documentation.  Failing that, what should I do to
make it available?

The weight private field should have no default value, but is intended
to be given one by the sub-class.  In my class-naif view of these
things, I should really make size% an interface and force the sub-class
to fill in the weight.  But I can't seem to figure out how to do that,
either.

Can someone explain to me the proper way to deal with these issues?  Any
advice welcome.

Many thanks,

 -tom

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