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We present a principled modular approach to the development of construction and verification tools for imperative programs, in which the control flow and the data flow are cleanly separated. Our simplest verification tool uses Kleene algebra with tests for the control flow of while-programs and their standard relational semantics for the data flow. It is expanded to a basic program construction tool by adding an operation for the specification statement and one single axiom. To include recursive procedures, Kleene algebras with tests are expanded further to quantales with tests. In this more expressive setting, iteration and the specification statement can be defined explicitly and stronger program transformation rules can be derived. Programming our approach in the Isabelle/HOL interactive theorem prover yields simple lightweight mathematical components as well as program construction and verification tools …