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
Response topography
The type of controlling variable (stimulus or establishing operation)
The specific number of multiple variables
Whether or not the current response is followed by reinforcement