Modernized Algol introduces the concept of block structurce, i.e. the scope of assignables. the stack-like behavior of assignables is a characteristic feature of what are known as Algol-like languages.
the language (Modernized Algol) maintains a sharp distinction between variables and assignables. variables are introduced by lambda-abstraction and given meaning by substitution. Assignables are introduced by a declaration and are given meaning by mutation (assignment and retrivial of their contents). Expressions evaluate to values and have no effect on assignable. commnads are executed for their effect on assignables and return a value. composition of commands not only sequences their execution order, but also passes the value returned by the first to the second before it is executed 引自第304页