def doOpen(opentype):
    """opentype is a tuple. Return None if more index tokens are
        required. Check to see if this kind of child object conforms to the
        constraint, raise Violation if not. Create a new Unslicer (usually
        by delegating to self.parent.doOpen, up to the RootUnslicer). Set a
        constraint on the child unslicer, if any.
        """
