On the definitional method of standard PL/I

In order to define the semantics of PL/I in a form which is both precise and readable, a method of definition has been developed which employs an abstract machine operating on tree-structured data. The classes of trees involved are defined by formal grammars, while the algorithms governing the behaviour of the machine are expressed semi-formally in prose. This algorithmic metalanguage is itself intended to have largely intuitive semantics, but these are made more precise by an indication of how they could be supported at a lower level of detail within the abstract machine.