Return a Deferred which will fire with the customized version of the
        resource being located.

def locateChild(self, ctx, segments):
        """
        Return a Deferred which will fire with the customized version of the
        resource being located.
        """
        D = defer.maybeDeferred(
            self.currentResource.locateChild, ctx, segments)

        def finishLocating((nextRes, nextPath)):
            custom = ixmantissa.ICustomizable(nextRes, None)
            if custom is not None:
                return (custom.customizeFor(self.forWho), nextPath)
            self.currentResource = nextRes
            if nextRes is None:
                return (nextRes, nextPath)
            return (_CustomizingResource(nextRes, self.forWho), nextPath)

        return D.addCallback(finishLocating)