def dereference(self):
        """Instruct all children to perform dereferencing."""
        all = []
        indexes = {}
        for child in self.children:
            child.content(all)
        dependencies = {}
        for x in all:
            x.qualify()
            midx, deps = x.dependencies()
            dependencies[x] = deps
            indexes[x] = midx
        for x, deps in dependency_sort(dependencies):
            midx = indexes.get(x)
            if midx is None:
                continue
            d = deps[midx]
            log.debug("(%s) merging %s <== %s", self.tns[1], Repr(x), Repr(d))
            x.merge(d)
Instruct all children to perform dereferencing.