Pitfalls and Subtleties of Macros
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   In this section we describe some special rules that apply to macros
and macro expansion, and point out certain cases in which the rules have
counterintuitive consequences that you must watch out for.

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Misnesting

Macros can contain unmatched parentheses.

Macro Parentheses

Why apparently superfluous parentheses may be necessary to avoid incorrect grouping.

Swallow Semicolon

Macros that look like functions but expand into compound statements.

Side Effects

Unsafe macros that cause trouble when arguments contain side effects.

Self-Reference

Macros whose definitions use the macros' own names.

Argument Prescan

Actual arguments are checked for macro calls before they are substituted.

Cascaded Macros

Macros whose definitions use other macros.

Newlines in Args

Sometimes line numbers get confused.