Challenge 2: A Header Guard, and What Breaks Without One — Possible Solution
====================================================================

point.h:

#ifndef POINT_H
#define POINT_H

typedef struct {
    int x;
    int y;
} Point;

#endif

Using it (deliberately including it twice from the same file, to
demonstrate the guard working):

main.c:

#include "point.h"
#include "point.h"   // included again -- the guard prevents a problem

int main() {
    Point p = {1, 2};
    return 0;
}

With the guard in place, this compiles fine: the first #include pastes
point.h's contents and defines POINT_H; the second #include's #ifndef
POINT_H check sees POINT_H is already defined and skips pasting the
contents a second time entirely.

What would happen WITHOUT the guard (removing #ifndef/#define/#endif
entirely): both #include lines would paste the full typedef struct
{...} Point; definition into main.c, verbatim, twice. The compiler
would then see Point defined twice in the same translation unit,
producing a "redefinition of typedef 'Point'" compile error (the exact
category of error the chapter names -- "duplicate definitions... a
real compile error") -- the program would fail to compile at all, not
merely produce a warning.

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates the guard actually preventing the duplicate-paste
scenario (two #include lines, one working correctly), and explains
precisely what error the compiler produces without the guard
(duplicate struct/typedef definition, a compile error) rather than a
vague "it breaks."