Critical section review

[Neil Jerram]

OK, to get the ball rolling and fill in my understanding: 
scm_make_memoized() in debug.c has:

  SCM_CRITICAL_SECTION_START;
  SCM_NEWSMOB (z, SCM_UNPACK (exp), SCM_UNPACK (env));
  SCM_NEWSMOB (ans, scm_tc16_memoized, SCM_UNPACK (z));
  SCM_CRITICAL_SECTION_END;

So:

- why do we need a critical section here?

- given that there is a critical section here, isn't there a problem 
that one of the SCM_NEWSMOBs could throw a memory error?

(I have read what it says in the manual on critical sections, by the way.)

Regards,
        Neil