The purpose of gh_allow_ints()?

[9]

I got a guile snapshot of would-be guile version 1.6
and read the tutorial.  And I found gh_allow_ints()
in the example code, `c_builtins.c'.

SCM c_factorial(SCM s_n)
{
  int i;
  unsigned long result = 1, n;

  /* omitted */

  gh_allow_ints()
  return gh_ulong2scm(result);
}

There was no gh_allow_ints() in the old tutorial of
guile, if I correct.

1. Can anyone explain the purpose of gh_allow_ints()?

2. And on what condition do I have to call gh_allow_ints()?

3. Seems like that gh_allow_ints() and gh_defer_ints()
   are used in pairs, right?

Included reference (guile.info) says that gh_allow_ints()
enable Scheme's flow control, and it seems that
it is connected with some `re-entrance' issue, but I
can hardly understand.

Thanx.

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