StmObjCtxCellView
final class StmObjCtxCellView[T <: Txn[T]](attr: Attr[T], key: String) extends AbstractCtxCellView[T, Obj[T]]
A CellView[T, Option[LObj[T]] built from a Context.
This is very similar to StmObjAttrMapCellView, which is built for the
attribute map of a lucre.Obj instead of a context, but the two classes have to be distinguished
because lucre.Obj.AttrMap must be put into a handle, and because there
a special case exists when it used instead an LObj expression.
trait Source[T, Option[Obj[T]]]
trait Observable[T, Option[Obj[T]]]
class Object
trait Matchable
class Any