The poem is about death, and therefore, one source of strength is a thematic source that strengthens any response form about death. A second source of control is the intraverbal relationship between “chimney sweepers” and “dust.” A final controlling variable is the formal (rhyming) control between the responses “must” and “dust.”
A situation with the following features:
There is a single response form
The response is simultaneously strengthened by two or more controlling variables