HxOp
maintains all the state for the current request, and provides the interface
for creating and consuming a document. The original HxOp wraps
the WebOp
for the current request. Sub-views should be given a new HxOp from
the current op via the HxOp.make() method. See 'Writing Reusable
HxViews' in HxView.
Note: There should always be a one-to-one mapping of HxOps to
HxViews.