Two armed IF is essential.

[Jonathan A Rees <JAR@MIT-MC>]

    Date: 22 Apr 1985 18:36:48-EST
    From: linus!ramsdell at Mitre-Bedford

    One-armed IF may change from essential to optional status,
    but please don't change two-armed IF.

I think this was what Will intended to write.  The terminology is
confusing, since an IF with N arms has N+1 subforms, and he probably
wrote arm and meant subform.

So:  (IF test consequent alternate)  essential
     (IF test consequent)	     optional

Jonathan