Models specified in the language of basic protocols are considered. These models are attribute transition systems, and their states are defined by formulas of multisort first-order predicate calculus over system attributes. Attributes of simple numeric and symbolic types, functional types, and queues are allowed. Assignment operators, queue update operators, and arbitrary formulas are used in postconditions of basic protocols. To pass from one state to another, a predicate transformer is constructed as a function of formula transformation. The following main property of the predicate transformer is proved: it calculates the strongest postcondition for symbolic states.