Imagine what happens when someone says

    foreach(x, get_next(queue)) f(x);

To solve this problem, with GCC, you have the option of defining foreach as

    { typeof (xs) _xs = xs; \
      for (var x = iter_start(_xs); x = iter_next(_xs); x != iter_end(_xs))

but then you need a corresponding endforeach to apply the matching }. If you just leave off the opening {, then you end up with a mysterious compile error on

    if (ready(queue)) foreach(x, get_next(queue)) f(x);

since the declaration of _xs is not a statement.

And this kind of thing is why a "macro system" means something very different in C and in Lisp, and why Brad Cox wrote a compiler in 1983 instead of a macro library.