mred:add-preference-callback

• (mred:add-preference-callback p f) -> a thunk
      p : symbol
      f : a function of two arguments, a symbol and any scheme value, which returns #t or #f

  mred:add-prefererence-callback adds a callback which is called with a symbol naming a preference and it's value, when the preference changes. If the callback function returns #f, the preference is not changed. mred:add-preference-callback returns a thunk, which when invoked, removes the callback from this preference.

  The callbacks will be called in the order in which they were added.

  If you are adding a callback for a preference that requires marshalling and unmarshalling, you must set the marshalling and unmarshalling functions by calling mred:set-preference-un/marshall before adding a callback.

  This function raises mred:exn:unknown-preference if the preference has not been set.