I am a stand-in for an object that is out on the disk. The object that is out on the disk is the head of a tree of objects that are out. See SqueakPage.

When any message is sent to me, I don't understand it, and bring in my true object. I become myself with the objects and resend the message.

I may not represent the object nil.
The file is represented as a url, and that url may point at any file on the net.

page is a SqueakPage.
If the cache already has an object, widely in use, that claims to be the object for my url, what do I do? I can't become him, since others believe that he is the true object. Run through memory and replace refs to me with refs to him. Be careful not to trigger a fault. Become me to a string, then find pointers and replace?

[[[They don't want to end up holding an ObjectOut. (would oscillate back and forth) This is a problem. A user could bring in two trees that both refer to a 3rd url. (check with cache before installing any new ObjectOut) Two trees could be written to the same url.
Or, I remain an ObjectOut, and keep getting notUnderstood, and keep returning the other guy.
Or I smash the cache, and install MY page and object. Other guy is a copy -- still in, but with no place in the cache. When we both write to the same url, there will be trouble.] No -- search and replace.]]]

	basics
	isInMemory xxxClass xxxClone xxxInstVarAt: xxxInstVarAt:put:

	fetch from disk
	doesNotUnderstand: xxxFetch xxxFixup xxxSetUrl:page:

	object storage
	comeFullyUpOnReload: objectForDataStream: readDataFrom:size: storeDataOn: veryDeepCopyWith:

	access	top
	sqkPage
	url
	url:
	xxxReset mark as never brought in

	basics	top
	isInMemory We are a place holder for an object that is out.
	xxxClass Primitive. Answer the object which is the receiver's class. Essential. See Object documentation whatIsAPrimitive.
	xxxClone
	xxxInstVarAt: Primitive. Answer a fixed variable in an object. The numbering of the variables corresponds to the named instance variables. Fail if the index is not an Integer or is not the index of a fixed variable. Essential. See Object documentation whatIsAPrimitive.
	xxxInstVarAt:put: Primitive. Store a value into a fixed variable in the receiver. The numbering of the variables corresponds to the named instance variables. Fail if the index is not an Integer or is not the index of a fixed variable. Answer the value stored as the result. Using this message violates the principle that each object has sovereign control over the storing of values into its instance variables. Essential. See Object documentation whatIsAPrimitive.

	fetch from disk	top
	doesNotUnderstand: Bring in the object, install, then resend aMessage
	xxxFetch Bring in my object and replace all references to me with references to him. First try looking up my url in the pageCache. Then try the page (and install it, under its url). Then start from scratch with the url.
	xxxFixup There is already an object in memory for my url. All pointers to me need to be pointers to him. Can't use become, because other pointers to him must stay valid.
	xxxSetUrl:page:

	object storage	top
	comeFullyUpOnReload: Normally this read-in object is exactly what we want to store. Try to dock first. If it is here already, use that one.
	objectForDataStream: Return an object to store on a data stream (externalize myself).
	readDataFrom:size: Make self be an object based on the contents of aDataStream, which was generated by the object's storeDataOn: method. Return self.
	storeDataOn: Store myself on a DataStream. See also objectToStoreOnDataStream. must send 'aDataStream beginInstance:size:'
	veryDeepCopyWith: Copy me and the entire tree of objects I point to. An object in the tree twice is copied once, and both references point to him. deepCopier holds a dictionary of objects we have seen. Some classes refuse to be copied. Some classes are picky about which fields get deep copied.