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 that is, all arguments to a procedure are fully evaluated before an attempt is made to apply the procedure to the arguments. (It is necessary to state this explicitly here, as it is not inherent in the form of the meta-circular definition. See [Reynolds] for an explication of this problem.)

The driver loop (see Figure 1) is conceptually started by a request to invoke DRIVER with no arguments. Its task is to first print the message "LITHP ITH LITHTENING" (a tradition of sorts) and then invoke DRIVER-LOOP. The expression  is intended to represent a constant list structure, containing definitions of primitive procedures, to be supplied to DRIVER-LOOP.

Figure 1 Top Level Driver Loop for a Recursion Equations Interpreter

DRIVER-LOOP reads an S-expression from the input stream and passes it, along with the current procedure definitions, to DRIVER-LOOP-1. This procedure in turn determines whether the input S-expression is a definition. If it is, then it uses BIND (described below) to produce an augmented set of procedure definitions, prints the name of the defined procedure, and calls DRIVER-LOOP to repeat the process. The augmented set of procedures is passed to DRIVER-LOOP, and so the variable PROCEDURES always contains all the accumulated definitions ever read. If the input S-expression is not a definition, then it is given to the evaluator EVAL, whose purpose is to determine the values of expressions. {Note Value Quibble} The set of currently defined procedures is also passed to EVAL.

The process carried on by the driver loop is often called the "top level"; all user programs and requests are run "under" it. The growing set of procedure definitions is called the "top-level environment"; this environment changes in the course of the user interaction, and contains the state of the machine as perceived by the user. It is within this environment that user programs are executed.