Common Lisp the Language, 2nd Edition

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26.1. Introduction

A loop is a series of expressions that are executed one or more times, a process known as iteration. The Loop Facility defines a variety of useful methods, indicated by loop keywords, to iterate and to accumulate values in a loop.

Loop keywords are not true Common Lisp keywords; they are symbols that are recognized by the Loop Facility and that provide such capabilities as controlling the direction of iteration, accumulating values inside the body of a loop, and evaluating expressions that precede or follow the loop body. If you do not use any loop keywords, the Loop Facility simply executes the loop body repeatedly.