all reusability have limits! usually work is carried out to satisfy a set of requirements. as long as requirements are matching old ones in a different task then the product can be reused but in case there is deviation then alterations will be required to address the differing bits. we may predict future requirements and address those too through generalization but that must have limits as addressing predictions is by definition not certain, may need refactoring still in the future while using actual resources. even the stl of c++ needs rework as it does not address certain requirements or address outdated ones. no code will be reusable completely all the time, in fact only reusable if specific circumstances match. generally speaking generalization is impossible.