In Chapter 1, I asserted that the grammar of Lisp is uniform: every expression is a list, beginning with a verb, and followed by some arguments. Evaluation proceeds from left to right, and every element of the list must be evaluated before evaluating the list itself. Yet we just saw, at the end of Sequences, an expression which seemed to violate these rules.
Clearly, this is not the whole story.
There is another phase to evaluating an expression; one which takes place before the rules we’ve followed so far. That process is called macro-expansion. During macro-expansion, the code itself is restructured according to some set of rules–rules which you, the programmer, can define.