Roof is part of Cave. Cave can be used for Darkness. 
Poor is related to Rich. Rich is related to Degree. Degree is related to Part. Part is related to Angle. 
Roof --PartOf--> Cave --UsedFor--> Darkness 
Poor <--RelatedTo--> Rich <--RelatedTo--> Degree <--RelatedTo--> Part <--RelatedTo--> Angle 