“Towards a Theory of Recursive Processors”

In order to define what we mean by processor, we must describe a primitive undefined processor called the initial control processor, and a primitive command which may be assumed to be executable ab initio. The initial control processor causes a statement in some other processor to be moved from the object level to the command level, and then causes the new-formed command to be executed. The primitive command which cannot be defined, but merely described, is the command append. This command is of the form “append &Oslash₁...&Oslash_n to the list of acceptable commands and give them interpretations θ₁...&thgr_n respectively” and serves to enable the control processor to interpret new commands. The language in which the &Oslash's and &thgr/s are written is undefined; we shall refer to the initial control processor as augmented by appends as the augmented control processor.