2972	 Proof Techniques for Hierarchically Structured Programs	 A method for describing and structuring programs that simplifies proofs of their correctness is presented. The method formally represents a program in terms of levels of abstraction each level of which can be described by a self-contained nonprocedural specification. The proofs like the programs are structured by levels. Although only manual proofs are described in the paper the method is also applicable to semi-automatic and automatic proofs. Preliminary results are encouraging indicating that the method can be applied to large programs such as operating systems. hierarchical structure program verification structured programming formal specification abstraction and programming methodology
