Assignment and Procedure Call Proof Rules

The multiple assignment statement is defined in full generality—including assignment to subscripted variables and record fields—using the “axiomatic” approach of Hoare. Proof rules are developed for calls of procedures using global variables, var parameters, result parameters, and value parameters, using the idea of multiple assignment to provide understanding. An attempt is made to clarify some issues that have arisen concerning the use of rules of inference to aid in generating “verification conditions” in mechanical verifiers and the use of logical variables to denote initial values of program variables.